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/WiFiServerSecureBearSSL.h

@@ -0,0 +1,76 @@
+/*
+  WiFiServerBearSSL.h - Library for Arduino ESP8266
+  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 wifiserverbearssl_h
+#define wifiserverbearssl_h
+
+#include "WiFiServer.h"
+#include "WiFiClientSecureBearSSL.h"
+#include "BearSSLHelpers.h"
+#include <bearssl/bearssl.h>
+
+namespace BearSSL {
+
+class WiFiClientSecure;
+
+class WiFiServerSecure : public WiFiServer {
+  public:
+    WiFiServerSecure(IPAddress addr, uint16_t port);
+    WiFiServerSecure(uint16_t port);
+    virtual ~WiFiServerSecure();
+
+    // Override the default buffer sizes, if you know what you're doing...
+    void setBufferSizes(int recv, int xmit) {
+      _iobuf_in_size = recv;
+      _iobuf_out_size = xmit;
+    }
+
+    // Set the server's RSA key and x509 certificate (required, pick one).
+    // Caller needs to preserve the chain and key throughout the life of the server.
+    void setRSACert(const BearSSLX509List *chain, const BearSSLPrivateKey *sk);
+    // Set the server's EC key and x509 certificate (required, pick one)
+    // Caller needs to preserve the chain and key throughout the life of the server.
+    void setECCert(const BearSSLX509List *chain, unsigned cert_issuer_key_type, const BearSSLPrivateKey *sk);
+
+    // Require client certificates validated against the passed in x509 trust anchor
+    // Caller needs to preserve the cert throughout the life of the server.
+    void setClientTrustAnchor(const BearSSLX509List *client_CA_ta) {
+      _client_CA_ta = client_CA_ta;
+    }
+
+    // If awaiting connection available and authenticated (i.e. client cert), return it.
+    WiFiClientSecure available(uint8_t* status = NULL);
+
+    // Compatibility with axTLS interface
+    void setServerKeyAndCert(const uint8_t *key, int keyLen, const uint8_t *cert, int certLen);
+    void setServerKeyAndCert_P(const uint8_t *key, int keyLen, const uint8_t *cert, int certLen);
+
+  private:
+    const BearSSLX509List *_chain = nullptr;
+    unsigned _cert_issuer_key_type = 0;
+    const BearSSLPrivateKey *_sk = nullptr;
+    int _iobuf_in_size = BR_SSL_BUFSIZE_INPUT;
+    int _iobuf_out_size = 837;
+    const BearSSLX509List *_client_CA_ta = nullptr;
+    bool _deleteChainAndKey = false;
+};
+
+};
+
+#endif