Dependency automation seems a bit confused in these makefiles.
Clean it up following ancient sage wisdom here:
http://mad-scientist.net/make/autodep.html
In short, use -MMD to generate dependency files for each .o target,
and then use a sed script to extend that to avoiding "missing file"
dependencies when headers are renamed/removed.
Also, make everything dependent on makefiles other than our
autodependency (*.P) makefiles.
Make the BUILDDIR an order-only prerequisite for our targets so any
changed file therein does not cause all other files to appear to be
out of date.
Also, some makefile cleanup:
- Remove obsolete 'depend' target.
- Move AVR-specific targets to AVR makefile.
- Add TARGET variable to identify target to make and to clean.
- Tidy up dependency make.
Phil Hord