efi_loader: file: support creating a directory

In efi world, there is no obvious "mkdir" interface, instead, Open()
with EFI_FILE_MODE_CREATE in mode parameter and EFI_FILE_DIRECTORY
in attributes parameter creates a directory.

In this patch, efi_file_open() is extended so as to accept such
a combination of parameters and call u-boot's mkdir interface for
expected action.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>