This is a first step in protecting against fault injection attacks: the
attacker can no longer change failure into success by flipping a single bit.
Additional steps are needed to prevent other attacks (instruction skip etc)
and will be the object of future commits.
The return value of uECC_vli_equal() should be protected as well, which will
be done in a future commit as well.
Add a basic unit test for the ECDSA part of the tinycrypt.
It generates keys, signs and verifies. Modified from tinycrypt
tests found in tinycrypt-repository.
