Add BearSSL client and server, support true bidir, lower memory, modern SSL (#4273)

BearSSL (https://www.bearssl.org) is a TLS(SSL) library written by
Thomas Pornin that is optimized for lower-memory embedded systems
like the ESP8266. It supports a wide variety of modern ciphers and
is unique in that it doesn't perform any memory allocations during
operation (which is the unfortunate bane of the current axTLS).

BearSSL is also absolutely focused on security and by default performs
all its security checks on x.509 certificates during the connection
phase (but if you want to be insecure and dangerous, that's possible
too).

While it does support unidirectional SSL buffers, like axTLS,
as implemented the ESP8266 wrappers only support bidirectional
buffers. These bidirectional buffers avoid deadlocks in protocols
which don't have well separated receive and transmit periods.

This patch adds several classes which allow connecting to TLS servers
using this library in almost the same way as axTLS:
BearSSL::WiFiClientSecure - WiFiClient that supports TLS
BearSSL::WiFiServerSecure - WiFiServer supporting TLS and client certs

It also introduces objects for PEM/DER encoded keys and certificates:
BearSSLX509List - x.509 Certificate (list) for general use
BearSSLPrivateKey - RSA or EC private key
BearSSLPublicKey - RSA or EC public key (i.e. from a public website)

Finally, it adds a Certificate Authority store object which lets
BearSSL access a set of trusted CA certificates on SPIFFS to allow it
to verify the identity of any remote site on the Internet, without
requiring RAM except for the single matching certificate.
CertStoreSPIFFSBearSSL - Certificate store utility

Client certificates are supported for the BearSSL::WiFiClientSecure, and
what's more the BearSSL::WiFiServerSecure can also *require* remote clients
to have a trusted certificate signed by a specific CA (or yourself with
self-signing CAs).

Maximum Fragment Length Negotiation probing and usage are supported, but
be aware that most sites on the Internet don't support it yet.  When
available, you can reduce the memory footprint of the SSL client or server
dramatically (i.e. down to 2-8KB vs. the ~22KB required for a full 16K
receive fragment and 512b send fragment).  You can also manually set a
smaller fragment size and guarantee at your protocol level all data will
fit within it.

Examples are included to show the usage of these new features.

axTLS has been moved to its own namespace, "axtls".  A default "using"
clause allows existing apps to run using axTLS without any changes.

The BearSSL::WiFi{client,server}Secure implements the axTLS
client/server API which lets many end user applications take advantage
of BearSSL with few or no changes.

The BearSSL static library used presently is stored at
https://github.com/earlephilhower/bearssl-esp8266 and can be built
using the standard ESP8266 toolchain.

libraries/ESP8266WiFi/src/BearSSLHelpers.h

@@ -0,0 +1,122 @@
+/*
+  WiFiClientBearSSL- SSL client/server for esp8266 using BearSSL libraries
+  - Mostly compatible with Arduino WiFi shield library and standard
+    WiFiClient/ServerSecure (except for certificate handling).
+
+  Copyright (c) 2018 Earle F. Philhower, III
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+#ifndef _BEARSSLHELPERS_H
+#define _BEARSSLHELPERS_H
+
+#include <bearssl/bearssl.h>
+
+// Internal opaque structures, not needed by user applications
+namespace brssl {
+  class public_key;
+  class private_key;
+};
+
+// Holds either a single public RSA or EC key for use when BearSSL wants a pubkey.
+// Copies all associated data so no need to keep input PEM/DER keys.
+// All inputs can be either in RAM or PROGMEM.
+class BearSSLPublicKey {
+  public:
+    BearSSLPublicKey();
+    BearSSLPublicKey(const char *pemKey);
+    BearSSLPublicKey(const uint8_t *derKey, size_t derLen);
+    ~BearSSLPublicKey();
+
+    bool parse(const char *pemKey);
+    bool parse(const uint8_t *derKey, size_t derLen);
+
+    // Accessors for internal use, not needed by apps
+    bool isRSA() const;
+    bool isEC() const;
+    const br_rsa_public_key *getRSA() const;
+    const br_ec_public_key *getEC() const;
+
+    // Disable the copy constructor, we're pointer based
+    BearSSLPublicKey(const BearSSLPublicKey& that) = delete;
+
+  private:
+    brssl::public_key *_key;
+};
+
+// Holds either a single private RSA or EC key for use when BearSSL wants a secretkey.
+// Copies all associated data so no need to keep input PEM/DER keys.
+// All inputs can be either in RAM or PROGMEM.
+class BearSSLPrivateKey {
+  public:
+    BearSSLPrivateKey();
+    BearSSLPrivateKey(const char *pemKey);
+    BearSSLPrivateKey(const uint8_t *derKey, size_t derLen);
+    ~BearSSLPrivateKey();
+
+    bool parse(const char *pemKey);
+    bool parse(const uint8_t *derKey, size_t derLen);
+
+    // Accessors for internal use, not needed by apps
+    bool isRSA() const;
+    bool isEC() const;
+    const br_rsa_private_key *getRSA() const;
+    const br_ec_private_key *getEC() const;
+
+    // Disable the copy constructor, we're pointer based
+    BearSSLPrivateKey(const BearSSLPrivateKey& that) = delete;
+
+  private:
+    brssl::private_key *_key;
+};
+
+// Holds one or more X.509 certificates and associated trust anchors for
+// use whenever BearSSL needs a cert or TA.  May want to have multiple
+// certs for things like a series of trusted CAs (but check the CertStore class
+// for a more memory efficient way).
+// Copies all associated data so no need to keep input PEM/DER certs.
+// All inputs can be either in RAM or PROGMEM.
+class BearSSLX509List {
+  public:
+    BearSSLX509List();
+    BearSSLX509List(const char *pemCert);
+    BearSSLX509List(const uint8_t *derCert, size_t derLen);
+    ~BearSSLX509List();
+
+    bool append(const char *pemCert);
+    bool append(const uint8_t *derCert, size_t derLen);
+
+    // Accessors
+    size_t getCount() const {
+      return _count;
+    }
+    const br_x509_certificate *getX509Certs() const {
+      return _cert;
+    }
+    const br_x509_trust_anchor *getTrustAnchors() const {
+      return _ta;
+    }
+
+    // Disable the copy constructor, we're pointer based
+    BearSSLX509List(const BearSSLX509List& that) = delete;
+
+  private:
+    size_t _count;
+    br_x509_certificate *_cert;
+    br_x509_trust_anchor *_ta;
+};
+
+#endif