Move BearSSL from STACK_PROXY to a real, thunked 2nd stack (#5168)

* Update to BearSSL 0.6+ release, add AES_CCM modes

Pull in latest BearSSL head (0.6 + minor additions) release and add AES_CCM
modes to the encryption options.

* Enable the aes_ccm initialization in client/server

* Initial attempt

* Working code with second stack thunking

* Remove #ifdefs in .S file, not needed.

* Clean up thunks and remove separate stack flag

* Fix PIO assembler errors

* Remove #ifdef code changes, ensure same code as PC

Remove "#ifdef ESP8266;...;#else;...;#endif" brackets in BearSSL to
ensure the host-tested code is the same as the ESP8266-run code.

* Move to latest BearSSL w/EC progmem savings

* Merge with master

* Add br_thunk_* calls to do ref counting, painting

Add reference counting br_thunk_add/del_ref() to replace stack handling code
in the class.

Add in stack painting and max usage calculation.

* Add in postmortem stack dump hooks

When a crash occurs while in the second stack, dump the BSSL stack and
then also the stack that it was called from (either cont or sys).

* Update stack dump to match decoder expectations

* Move thunk to code core for linkiage

The thunk code needs to be visible to the core routines, so move it to the
cores/esp8266 directory.  Probably need to refactor the stack setup and the
bearssl portion to avoid dependency on bearssl libs in cores/esp8266

* Add 2nd stack dump utility routine

* Refactor once more, update stack size, add stress

Make stack_thunks generic, remove bearssl include inside of cores/esp8266.

Allocate the stack on a WiFiServerSecure object creation to avoid
fragmentation since we will need to allocate the stack to do any
connected work, anyway.

A stress test is now included which checks the total BearSSL second
stack usage for a variety of TLS handshake and certificate options
from badssl.org.

* Update to latest to-thunks branch

* Add BearSSL device test using stack stress

Run a series of SSL connection and transmission tests that stress
BearSSL and its stack usage to the device tests.

Modify device tests to include a possible SPIFFS generation and
upload when a make_spiffs.py file is present in a test directory.

* Use bearssl/master branch, not /to-thunks branch

Update to use the merged master branch of bearssl.  Should have no code
changes.

tests/README.md

@@ -114,6 +114,7 @@ Some tests need to connect to WiFi AP or to the PC running the tests. In the tes
 
 Environment variables can also be used to pass some information from the test code to the host side helper. To do that, test code can set an environment variable using `setenv` C function. Then the `teardown` host side helper can obtain the value of that variable using `request_env` function defined in `mock_decorators`.
 
+A SPIFFS filesystem may be generated on the host and uploade before a test by including a file called `make_spiffs.py` in the individual test directory.
 
 ### Building and running the tests