Built from master (commit 6e56ed129c9782ba050a5fbfbf4ac12335b230f7),
which has ati vgabios support merged (checkout master branch in
roms/seabios submodule, then run "make -C roms seavgabios-ati").
Temporary exception until the next seabios major version is
released (probably 1.13, fall 2019).