FWU: Add support for the FWU Multi Bank Update feature

The FWU Multi Bank Update feature supports updating firmware images
to one of multiple sets(also called banks) of images. The firmware
images are clubbed together in banks, with the system booting images
from the active bank. Information on the images such as which bank
they belong to is stored as part of the metadata structure, which is
stored on the same storage media as the firmware images on a dedicated
partition.

At the time of update, the metadata is read to identify the bank to
which the images need to be flashed(update bank). On a successful
update, the metadata is modified to set the updated bank as active
bank to subsequently boot from.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>