feat(stm32mp1): add logic to select the images to be booted

With the FWU multi bank boot feature enabled, the platform can boot
from one of the multiple banks(partitions) containing the firmware
images. The bank whose firmware components are to be booted is read
from the FWU metadata structure -- the image to be booted is thus
derived by reading the metadata.

Read the metadata and set the image spec of the corresponding image
type to point to the partition from which the image is to be booted.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: I3dfdc7e9202859e917ec4e1f7d1855aad42c6b70