bootutil: Allow SHA512 with TinyCrypt

The commit enables SHA512 support, for image hashing, with
TinyCrypt.
Although on 32bit machines the SHA256 will be faster than
SHA512, benefit of enabling the SHA512 is that you have only
one algorithm compiled in which reduces size of code.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>