bootutil: Replace hash with SHA384 when P384 is used

Currently all the hashing functionality is done with SHA256
but if we would like to use ECDSA-P384 that requires SHA384
as the hashing algorithm, but MCUboot is using SHA256
for image hashing and public key hashing. This commit modifies
the hashing operations to use SHA384 thus SHA256 can be omitted
which is beneficial from a code size standpoint.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: I59230f76f88e0b42ad6383b2c9b71b73f33d7dd7