Reimplement atomic code in inline assembly. This can improve
optimisation, and avoids potential architectural problems with using
LDREX/STREX intrinsics.
API further extended:
* Bitwise operations (fetch_and/fetch_or/fetch_xor)
* fetch_add and fetch_sub (like incr/decr, but returning old value -
aligning with C++11)
* compare_exchange_weak
* Explicit memory order specification
* Basic freestanding template overloads for C++
This gives our existing C implementation essentially all the functionality
needed by C++11.
An actual Atomic<T> template based upon these C functions could follow.
This also includes their Datagram counterparts: DTLSSocket and DTLSSocketWrapper.
Coverage missing for timer-related functionality, but this would require more advanced stub development.
The extra (d)tls_test_config.h is necessary for successful compilation without HW support for the mbedtls features in unittests.
I reused the mbedtls stub found in features/nanostack/coap-service/test/coap-service/unittest/stub/mbedtls_stub.c and amended it slightly with a few missing functions.