feat(fwu): add a function to pass metadata structure to platforms

Add a helper function to pass the metadata structure to the
platforms. Platforms can then read the metadata structure and pass the
boot index value, i.e. the bank(partition) from which the firmware
images were booted, to the Update Agent.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: I571179b9baa0fbc4d0f08d7a6e3b50c0c7165c5c