efi_loader: add firmware management protocol for FIT image

In this commit, a very simple firmware management protocol driver
is implemented. It will take a common FIT image firmware in a capsule
file and apply the data using dfu backend storage drivers via
update_fit() interface.

So "dfu_alt_info" variable should be properly set to specify a device
and location to be updated. Please read README.dfu.

Fit image is a common file format for firmware update on U-Boot, and
this protocol works neatly just as a wrapper for one.

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