zephyr: Support for HKDF/HMAC with SHA512

The commit adds CONFIG_BOOT_HMAC_SHA512 that enables MCUboot
configuration option MCUBOOT_HMAC_SHA512, that is used for
switching HKDF/HMAC in ECIES key exchange to SHA512,
from default SHA256.
This option, currently, is only available for ECIES-X25519
with PSA as crypto backend.

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