GDB attach regression

[netbub]

Because of changes made in this issue, the gdbserver on the ASA no longer sends "Remote debugging using /dev/ttyS0" over the serial line, which is what comm.py uses to signal that booting has finished.

I messed around a bit with getting gdbserver to respond to characters periodically sent over the serial line, but ran into issues getting that working. A somewhat simplistic fix that I tested is instead waiting for the "dosfsck(/dev/hda1) returned 0" string, sleeping for ten seconds, then breaking the while loop. Let me know if that works for you guys and I'll open a pull request.