no build instructions for ast27x0_bootrom.bin in roms/Makefile

pc-bios/README.md refers to the source of ast27x0_bootrom.bin, but there's no build instructions for this blob provided in roms/Makefile, which has build rules for all other ROM files.

The source is - apparently - available in roms/vbootrom/ (though not necessary of the correct commit ID - I haven't checked), there are other bootroms already built from it.

I think this is a serious enough issue to fix for 10.1 release, since we don't really have a way to reproduce this binary.