bios_emulator: Add Kconfig and adjust Makefile for SPL

The Kconfig for this is currently inside a particular board. Move it into
the correct place and allow use in SPL, so that video can be used there
if needed.

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