efi_loader: hook up in build environment

Now that we have all the bits and pieces ready for EFI payload loading
support, hook them up in Makefiles and KConfigs so that we can build.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
[trini: Enable only when we of OF_LIBFDT, disable on kwb and colibri_pxa270]
Signed-off-by: Tom Rini <trini@konsulko.com>