Z80 boot code

This assembles the boot binary. It requires the Z80 assembler
(B5) and cross compilation setup (B260). It requires some
constants to be set. See doc/bootstrap.txt for details.

RESERVED REGISTERS: At all times, IX points to RSP TOS and BC
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".

The boot binary is loaded in 2 parts. The first part, "decla-
rations", are loaded after xcomp, before xcomp overrides, with
"282 LOAD". The rest, after xcomp overrides, with "283 335
LOADR".