This allows creating *.checksums files instead of checking against
them when UPDATE_CHECKSUMS is set to True in bootstrap.cfg.
The checksums are also copied to /usr/src so they can be accessed
easily after the bootstrap completes.
- Rename sources to distfiles for clarity.
- Per sys(a/c) distfiles to reduce rootfs.py processing and reduce RAM
usage in sysa.
- Canonicalise early kaem mes/tcc files to kaem script conventions.
- Cleanup unused setup in python.
- This is much more standard and replaces /image in sysa and is the
standard in sysc (avoids many issues).
- GCC needs to have a file created for some unknown reason.
- Checksums updated.