efi_loader: return binary from efi_dp_from_lo()

For finding distro supplied device-trees we need to know from which device
we are booting. This can be identified via the device-path of the binary.

Up to now efi_dp_from_lo() only could return the initrd or fdt device-path.
Allow returning the binary device-path, too.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>