SeaBIOS: have coreboot pass the choice to run optionroms in parallel

Introduce the tunable CONFIG_SEABIOS_THREAD_OPTIONROMS.

Change-Id: Ifd4d9fca7316eb739ff184e54bdc1cdb0262f0c6
Signed-off-by: Idwer Vollering <vidwer@gmail.com>
Reviewed-on: http://review.coreboot.org/5443
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>