efi_loader: disk: install file system protocol to a whole disk

Currently, a whole disk without any partitions is not associated
with EFI_SIMPLE_FILE_SYSTEM_PROTOCOL. So even if it houses some
file system, there is a chance that we may not be able to access
it, particularly, when accesses are to be attempted after searching
that protocol against a device handle.

With this patch, EFI_SIMPLE_FILE_SYSTEM_PROTOCOL is installed
to such a disk if part_get_info() shows there is no partition
table installed on it.

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

Only if no partition table exists, check for a file system on disk level.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>