configs: Fix crash on coreboot x86

configs: Fix crash on coreboot x86

Booting u-boot as payload with coreboot's main branch is currently broken
since commit [1] on x86 as U-boot assumes the active GDT matches what
U-Boot would have installed in start16.S.

Make no assumptions and always load the GDT when building as coreboot
payload to make sure the segment registers are actually matching the GDT.

Fixes #GP seen when booting U-Boot as coreboot payload.

1: https://review.coreboot.org/c/coreboot/+/87255

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Link: https://review.coreboot.org/c/coreboot/+/87255
Reviewed-by: Tom Rini <trini@konsulko.com>