bootutil: Allow the usage of builtin keys in verification

Introduce a new MCUBOOT_BUILTIN_KEY option to enable the usage of
builtin keys for signature verification. This way the details of the key
handling mechanism are abstracted away from the boot code and this
responsibility is delegated to the given crypto library.
This is an alternative option to the existing MCUBOOT_HW_KEY feature,
however in this case we can entirely rely on key IDs and not only the
code, but also the image metadata does not contain any public key data.

Change-Id: Id01b67951310549b2734730c58bfa7210a2d5236
Signed-off-by: David Vincze <david.vincze@arm.com>