payloads/external/Makefile.inc: Fix SeaBIOS option regressions
Commit
14ca7407196 (Makefile.inc: Move adding SeaBIOS cbfs config files)
introduced various regressions that were not spotted during review.
TEST=Building with SEABIOS_THREAD_OPTIONROMS is working properly again.
Change-Id: I4de0b11747e3df8dd31a85160add129d8cc6bd8a
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47814
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>