Instead of letting each configuration taking care of RDLN$ and
"CollapseOS" prompt, move this to BOOT to simplify xcomp units.
Initialization source code is now only for driver initialization.
DO and LOOP now only compile words from stable ABI, which make them
suitable for xcomp. This will greatly simplify driver writing and
allow us to mostly avoid the low/high divide in drivers.
LEAVE is still xcomp-incompatible though.
This make us very very tight for "<0x100" limit for literal words.
Hopefully, we won't need more stuff in that area of the binary...
