BL2: Add Key ID to image for the built in keys

When MCUBOOT_BUILTIN_KEY is enabled, add the key id to the image
so that it can be parsed by mcuboot to identify the key used to
sign the image.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: I6872b8a88e0292f7964a57a0219e913d599f9f95
3 files changed