Add configs for the Latitude E6400

Tested the 4MiB ROMs but not the 8 or 16 MiB ones. This uses the same
board.cfg as the GM45 ThinkPads with an IFD+GBE from ich9gen.

Known issues:
- The internal keyboard does not work properly in GRUB. It seems like
  the keyboard controller is outputing set 1 (XT) scancodes, but GRUB
  is interpreting them as set 2 (AT) scancodes. This may also have
  something to do with scancode translation. However, the keyboard works
  fine in SeaBIOS and Linux. USB keyboards also work properly.
- The subsystem IDs in the GBE region are hardcoded for a Thinkpad in
  ich9gen, though this doesn't seem to cause issues in Linux. The vendor
  IFD and GBE region do have some differences from the generated
  binaries, though they do not appear to be critical.