efi_loader: find distro device-path for media devices

The auto-generated load options for media device do not contain a partition
node. We cannot expect the simple file protocol here.

Get the partition device-path via the loaded image protocol.

Fixes: e91b68fd ("efi_loader: load distro dtb in bootmgr")
Reported-by: E Shattow <lucent@gmail.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: E Shattow <lucent@gmail.com>