When we don't have a real device/image path, such as 'bootefi hello',
construct a mem-mapped device-path.

This fixes 'bootefi hello' after devicepath refactoring.

Fixes: 95c5553e ("efi_loader: refactor boot device and loaded_image handling")
Signed-off-by: NRob Clark <robdclark@gmail.com>
Acked-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: NAlexander Graf <agraf@suse.de>