efi_loader: capsule: add capsule_on_disk support

Capsule data can be loaded into the system either via UpdateCapsule
runtime service or files on a file system (of boot device).
The latter case is called "capsules on disk", and actual updates will
take place at the next boot time.

In this commit, we will support capsule on disk mechanism.

Please note that U-Boot itself has no notion of "boot device" and
all the capsule files to be executed will be detected only if they
are located in a specific directory, \EFI\UpdateCapsule, on a device
that is identified as a boot device by "BootXXXX" variables.

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