bootutil: Add SHA-512 support to Ed25519

When Ed25519 signatures are used, the bootutil_verify_sig responsible
for verifying a signature was expecting as argument the SHA-256 digest
of the firmware image. This commit slightly modifies this routine to
make possible to use Ed25519 with SHA-512 digests.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>