feat(fwu): add platform hook for getting the boot index

Add a platform hook for returning the boot index, i.e. the bank from
which the platform has booted the updatable firmware images. This
value will be passed to the Update Agent.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: Ic7bef21071c48cfc7b69c50e89df9ff758d95b00