boot0sio.s was repo-copied to boot0.S.
- Rename boot0ext.s to boot0ext.S, to stay consistent
with other preprocessed asm files around here, and
for better portability.
Repocopied by: joe
Merge boot0.s and boot0sio.s into boot0_512.s controlled by "#ifdef SIO".
Add Makefile magic to generate boot0.s and boot0sio.s from boot0_512.s.
The compile boot0 and boot0sio have unchanged MD5 checksums.