FWU: developerbox: read boot index from NOR flash

The FWU Multi Bank Update feature allows the platform to boot the
firmware images from one of the partitions(banks).
On the Developerbox, SCP-firmware running on the SCB(Cortex-M3)
passes the value of the boot index on the NOR flash.
Add a function to read the boot index value from the NOR flash.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>