* Updates driver library to v2.3.1 (2018q1) for bugfixes and convenience functions
* Provides library in correct format (2-byte wchar_t flag) for compiling with ARMCC (#6695 uncovered by #6577)
* Reverts to using a statically-allocated packet buffer since malloc is not thread-safe (and the asserts have been turned on)
mbed compile doesn't support two different FEATURE_X folders being merged, so we'll have to move our nanostack driver into the Nanostack folder for the time being.