[coreboot] memtest on lx on v3: working

[ron minnich]

turned out the problem was some weirdness between minicom/memtest on
serial. I resized the terminal window that minicom was in and memtest
has been running fine for almost 5 hours.

I also have mode cache enable around so that it happens in
disable_car. This helps a bit. But I still feel we're not caching ROM.

ron