Renamed sha256 functions to comply with new crypto module

This PR fixes the test projects for the sha256 hash algorithm to comply with RIOT PR#391. Functionnames for the algorithm are changed to lower case and the USEMODULE string was changed to crypto_sha236.

I further removed the stm32 extensions from the Makefiles as these are (or will be very soon) obsolete and are not working in the current state anyhow...