boards: x86: depend on CONFIG_BUILD_OUTPUT_EFI

Add a new Kconfig CONFIG_BUILD_OUTPUT_EFI and select that for boards
that want to generate an EFI application.
Make qemu_x86_64 also generate an EFI file, however do not enable this
by default yet.

Goal is to boot qemu using EFI to be able to test this path in the
future.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>