Add file_block_store

To help with FWU debug and test, a file_block_store is added,
enabling a standard file containing a disk image to be accessed
directly by an application. This will be used to verify decoding
and configuration using of a boot image file generated by the
firmware build system.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: I85a00dd0536810ce10510abf05cdcfa9ad0c7621
9 files changed