x86: Enable EFI loader support

Enable this so that EFI applications (notably grub) can be run under U-Boot
on x86 platforms.

At present the 'hello world' EFI application is not supported for the
qemu-x86_efi_payload64 board. That board builds a payload consisting of a
64-bit header and a 32-bit U-Boot, which is incompatible with the way the
EFI loader builds its EFI application. The following error is obtained:

x86_64-linux-ld.bfd: i386 architecture of input file
   `lib/efi_loader/helloworld.o' is incompatible with i386:x86-64 output

This could be corrected with additional Makefile rules. For now, this
feature is disabled for that board.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[agraf: drop hello kconfig bits]
Signed-off-by: Alexander Graf <agraf@suse.de>