efidebug: capsule: Add a command to update capsule on disk

Add a efidebug subcommand to initiate a firmware update using the efi
firmware management protocol(fmp) set_image routine.

The firmware update can be initiated through

'efidebug capsule disk-update'

This would locate the efi capsule file on the efi system partition,
and call the platform's set_image fmp routine to initiate the firmware
update.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>