Allow switch between newlib-nano and newlib

The makefiles still defaults to using newlib-nano to produce smaller
binaries but the user can set the NEWLIB_NANO to 0 and the standard
newlib C library will be used instead.  One of the reasons I have added
this support is for people who want 64-bit integer support from the
printf family of functions which newlib-nano does not have.
Update to latest mbedmicro/mbed commit 66+

https://github.com/mbedmicro/mbed/commit/96ea3db1b37e4f615584fba010d46f37056474ed