Z80 boot code

This assembles the boot binary. It requires the Z80 assembler
(B200) and cross compilation setup (B260). It also requires
these constants to be set:

RAMSTART: beginning address of RAM. This is where system
variables are placed. HERE is then placed at RAM+80 (ref B80).

RS_ADDR: to be set to the bottom address of the Return Stack.

PS_ADDR: top address of the Parameter stack (PS grows down-
wards). Allow space for stack underflow protection (B76).


                                                        (cont.)