FWU: Add checking the state of the running image

The Firmware Update partition reads the image_ok flag to check
whether the running image has been confirmed. The running image
state is set according to this flag in psa_fwu_query().

Change-Id: I9cf58b7d6f3b87a50e0ac6f926e30170bb01b8f1
Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>