configure: do not create roms/seabios/config.mak if SeaBIOS not present

If roms/seabios/Makefile is not present, the configure script
is not creating the roms/seabios directory anymore (commit
5dce7b8d, "configure: remove DIRS", 2021-12-18); thus, creating
roms/seabios/config.mak fails.

The easiest thing to do is to not create the file, since it will not
be used.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>