FWU: Add boot time checks as highlighted by the FWU specification

The FWU Multi Bank Update specification requires the Update Agent to
carry out certain checks at the time of platform boot. The Update
Agent is the component which is responsible for updating the firmware
components and maintaining and keeping the metadata in sync.

The spec requires that the Update Agent perform the following checks
at the time of boot
* Sanity check of both the metadata copies maintained by the platform.
* Get the boot index passed to U-Boot by the prior stage bootloader
  and use this value for metadata bookkeeping.
* Check if the system is booting in Trial State. If the system boots
  in the Trial State for more than a specified number of boot counts,
  change the Active Bank to be booting the platform from.

Call these checks through the main loop event at the time of platform
boot.

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