x86: Create a 32/64-bit selection for the app

x86: Create a 32/64-bit selection for the app

Most EFI implementations use 64-bit but U-Boot only supports running as
a 32-bit app at present. While efi-x86_payload64 does boot from 64-bit
UEFI it immediately changes back to 32-bit before starting U-Boot.

In order to support a 64-bit U-Boot app, update the Kconfig to add an
option for 32/64 bit. Update the prompt for the existing option so it is
clear it relates to the stub. Move both up to just under the choice that
controls them, since this looks better and the menu.

Use CONFIG_EFI_APP in the Makefile instead of CONFIG_TARGET_EFI_APP,
since the latter is specific to a single target and we will have two.

Memory size is set to 32MB for now so that it can run on qemu without
increasing the default memory size. We may need to increase the default
later.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>