Re-introduce assertions in critical.c

[AlessandroA]

This issue is to keep track of the changes introduced to critical.c in #98.

Assertions have been commented out in that file as a temporary workaround for ARMmbed/mbed-drivers#176, but will need to be put back when the bug is properly fixed.

@0xc0170 @bogdanm

[ciarmcom]

ARM Internal Ref: IOTSFW-2232