microdrive: move the work buffer somewhere better

it was at 0x4000 to help debug, move it offscreen
vdp1: Allow the VDP location to be defined not hard coded

Not everything with a 9918 in it is an MSX box...
mtx512: Initial port code

Not yet bootable.

Still to complete:
	CTC setup
	DART serial ports
	CP/M tools scripts to spit out a memotech floppy disk
mtx: link VIDEO in the right place
mtx: move things about to allow space for unpack

Otherwise we fall over ourselves
mtx: correct VDP

Ports 1 and 2 not 0 and 1
mtx: Initialize the VDP and use it for vt work

Currently this is only half working - we init the vdp but have no font. We
need to upload our own font as CP/M or other boot code might not be using
the VDP display.
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
fsck: bug fixes for big endian fsck on small endian box

Also change misc because a signed 16bit value swizzled gets 32bit extended
which tripped the sanity checker wrongly
mtx: Further work on the floppy driver

We now load the superblocks and find the inode etc for init

Note that the order on the virtual disk is odd with the second head after
the first full set of tracks
mtx: Set the crtc up properly
mtx: 80 column driver

Use the crtc to do hardware scrolling
mtx512: Add tools for converting fuzix fs images into mfloppy disk format
mtx512: undo that

Amazing what a floppy bug can look like

The disk order is just fine
mtx: mask sector count on side1

The MTX512 doesn't write sector labels 17-32 on the second side but recycles
1-16
mtx: clean up caching of page state

map_restore restored the map but not the cache of the write only port, this
then caused crashes if interrupt returns occurred during a user memory
access.

Use the helpers throughout for neatness
mtx: README

Explain the shortcut way to load the images