Add a file-backed bootloader interface for test purposes

Created by: chombourger

When your bootloader is not supported by swupdate, it may be convenient to have a persistent settings store put on a separate partition (not affected by firmware updates). This is really meant to be an intermediate solution as the hosting partition may get corrupted if the device isn't cleanly shut down.

Signed-off-by: Cedric Hombourger Cedric_Hombourger@mentor.com