bootutil/crypto: Move BOOTUTIL_CRYPTO_ECDSA_P256_HASH_SIZE into common

BOOTUTIL_CRYPTO_ECDSA_P256_HASH_SIZE is a universal define
that should be moved into a common area of the header
instead of repeating it for every abstraction that needs it

Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>
Change-Id: I21e7511218d0dafac8b7337715932c6564d7c3a1