vdp: Create a shared device area

We want this as there are MSX1 boxes with MSX2 memory mappers (or from our
viewpoint MSX2 boxes with MSX1 video). In addition it turns out that the VDP
used on the Memotech MTX series machines is basically the same beastie.
z80pack: fix comment
commonmem: add a standard Z80 commonmem to .include
commonmem: switch some platforms to using the new .include
pcw8256: Various fixes

Make sure we link the commonmem.s first
Use the standard commonmem
Move the written parts of common clear of the first 256 bytes (so swap can
work)
pcw8256: fix swap defines
pcw8256: fix ramtop definition
Makefile: remove comment - staying with uzi.lnk ordering for now