x86: Support a chained-boot development flow

Sometimes it is useful to jump into U-Boot directly from coreboot or UEFI
without any 16-bit init. This can help during development by allowing U-Boot
to avoid doing all the init required by the platform.

U-Boot expects its GDT to be set up correctly by its 16-bit code. If
coreboot doesn't do this (because it hasn't run the payload setup code yet)
then this won't happen.

In this case we cannot rely on the GDT settings. U-Boot will hang or crash
if these are wrong. Provide a development-only option to set up the GDT
correctly. This is just a hack so you can jump to U-Boot from any stage of
coreboot, not just at the end.

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