Z80 boot code

This assembles the boot binary. It requires the Z80 assembler
(B200) and cross compilation setup (B260).

On top of that, it requires RAMSTART to be defined as the
beginning address of RAM. This is where system variables are
placed. HERE is then placed at RAM+80 (ref B80).

We also need RS_ADDR to be set to the bottom address of the
Return Stack.

RESERVED REGISTERS: At all times, IX points to RSP TOS and IY
is IP. SP points to PSP TOS, but you can still use the stack\
in native code. you just have to make sure you've restored it
before "next".                                          (cont.)