efi_loader: capsule: support firmware update

A capsule tagged with the guid, EFI_FIRMWARE_MANAGEMENT_CAPSULE_ID_GUID,
is handled as a firmware update object.
What efi_update_capsule() basically does is to load any firmware management
protocol (or fmp) drivers contained in a capsule, find out an appropriate
fmp driver and then invoke its set_image() interface against each binary
in a capsule.
In this commit, however, loading drivers is not supported.

The result of applying a capsule is set to be stored in "CapsuleXXXX"
variable, but its implementation is deferred to a fmp driver.

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