haswell: only use txtmod seabios configuration

i found that with libre mrc, usb was broken in grub

however, it worked nicely in seabios

for our purposes, doing seabios-only roms in text mode
is best for now

i'm going to re-add mrc.bin, but for t440p_12mb_mrc
and w541_12mb_mrc, as new config names. the regular
t440p_12mb and w541_12mb will continue to use libre
mrc, but the _mrc ones will use mrc.bin and retain the
grub payload in board.cfg