vdp1: Allow mixed VDP/non VDP

We add a .equ which indicates whether the vdp1 driver should provide the
functions for vt.c or ones that can be wrapped. For MSX we are happy as we
are but for MTX512 we can support a VDP and a 6845 at the same time.
vt: Add support for switching vt state

We need this for the MTX512 but it will also be useful later for MSX, as the
MSX also has a load of VDP RAM that we can use to support multiple text
consoles just like Xenix and later Linux did on a PC.
mtx: Fix up the consoles and serial

We now have four tty devices, 1 is the 6845, 2 is the VDP, 3 and 4 the
serial ports. The serial isn't finished and the IRQ logic hasn't been tackled.

The keyboard works.
mtx: fix typo