Z80 boot code

This assembles the boot binary. It requires the Z80 assembler
(B200) and cross compilation setup (B260). It requires some
constants to be set. See B420 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".

STABLE ABI: The boot binary starts with a list of references.
The address of these references have to stay to those addr-
esses. The rest of the Collapse OS code depend on it. In fact,
up until 0x67, the (?br) wordref, pretty much everything has
to stay put.                                            (cont.)