efi_loader: support booting via short-form device-path

The boot manager must support loading from boot options using a short-form
device-path, e.g. one where the first element is a hard drive media path.

See '3.1.2 Load Options Processing' in UEFI specification version 2.9.

Fixes: 0e074d12 ("efi_loader: carve out efi_load_image_from_file()")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>