Reject unsupported Rev A firmware patching

[ericlewis]

Summary

• Stop advertising Rev A/AB as patch targets.
• Exit early with a clear message when Rev A is selected.
• Leave the existing Rev B path in place.

Why

Rev A still needs a real pointer patch before its loader can run. That code is not ready yet. The script used to make a file that looked patched even though Rev A would not actually start the loader.

Verification

• python3 -m py_compile combine.py
• printf A into combine.py and verified it exits with the Rev A unsupported error
• git diff --check

[ericlewis]

Replaced by #14 with the cleaned branch name.