z80: Add bits ready for any endian fixing we have to do
dragon: move the display to 0x6000

This gives us clear space below 0x6000 to possibly allow for bigger binaries
(but there are some nasties about changing how the vectors must work that
 won't be doable on a "real" Dragon ROM)

It's also the needed basis for 32K COCO and Dragon32 support given a banked
cartridge unit and banked call support.
dragon: Major updates

- Crazy SAM RAM mapping support
- Propogate vectors correctly to user pages
- Flesh out the tricks.s code as needed (mostly by deleting bits)
- Keyboard driver
- IRQ support now working

This is sufficient to boot to the bootdev prompt, type a device number
and fail to mount the file system.
Makefile: avoid soft registers on 6809 for now

(not that gcc is actually generating them for some reason anyway)