efi: Support building a u-boot-app.efi executable

Add support for building U-Boot as an EFI application with a .efi suffix.
This can be loaded by EFI provided that EFI has the same bit width (32-
or 64-bit) as U-Boot. This unfortunate limitation is imposed by EFI.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>