efi_loader: efi_setup_loaded_image() handle missing file name

This is a preparatory patch.

efi_dp_split_file_path() is used to create device_path and file_path
from file_path for efi_setup_loaded_image().
In a special case, however, of HARDWARE_DEVICE/MEMORY, it doesn't
work expectedly since this path doesn't contain any FILE_PATH sub-type.

This patch makes a workaround.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>

Adjust the logic such that for all paths that do no end on a media file
path we return NULL as file_path.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>