Tweaked 6847 video behaviour to be like a real Atom

Change-Id: Ie7ccf4729681805f896291be89fcdfd625049a67
Moved BBC A000 rom to #18000 so all Atom RomBox ROMs can be used for Atom Mode

Change-Id: Icfad8d34498e10fe8cd7be911f24877d82695c46
Rebuild the 128K ROM with SDDOS 3.25 at #00000, and with the BBC A000 at #18000

Change-Id: Id3af00ad6a5e9c492d69bd2b5efdcb56f36fecec