boot: Add hardware key support

This change enables the public key (used for image authentication) to
be removed from MCUboot and be appended to the image instead. In this
case the key or its hash must be provisioned to the device and MCUboot
must be able to retrieve the key-hash from the hardware to compare it
with the calculated hash of the public key from the image manifest in
order to verify its validity before image authentication.

The source of this change:
https://review.trustedfirmware.org/c/trusted-firmware-m/+/1581



Change-Id: I36fe699732e0e4c113eaed331c22e707c722ed6e
Signed-off-by: David Vincze <david.vincze@linaro.org>