README: update
font6x8: 6 by 8 font courtesy of LinuxLCD
font8x8: Shrink font, compensate for base change

It's dumb having the small font 128 symbols long when the first 32 are
weird stuff. Make it 96, and shift by 32 chars when printing. Saves us 256
bytes
font6x8: Add to Makefile

Build the 6x8 font when needed
font6x8: Include config.h
mtx: allow some more space during boot by moving _DISCARD a bit
z80: note assumption in commonmem sizing
mtx: correct pagemap loading
mtx: tty fixes

Update comments
Fix the F1/F2 handling
Lock interrupts off to avoid an interrupt messing up a tty update by switching
mid update. This really needs a better model of processing, eg deferring the
echoed character if mid processing
vt: don't vanish the cursor on vt switches

This will need revisiting because the vt implementations mostly keep a single
'what was under the cursor' and 'cursor address'. We may need to make these
live in a standard place and save them too.
mtx: include a 6x8 font and upload it into the VDP

We can't it seems rely upon the firmware to have done this for us.