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.
This allows to use makefile rather than kaem script for building sed.
Makefiles for musl and mes are unified into one makefile with some
conditional code.
- 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.
