Add shortcuts to Makefile-example for regressiontests and program
so it can more easily be used as-is.
Remove the comment suggesting to use Makefile-AVR directly instead
since it is no longer more convenient, at least for the original
reason.
- "Traumflug" and "Markus Hitter" are the same, mention him only
once.
- Add more common F_CPU choices in comments.
- Hint to another choice in Makefile-example.