DO NOT MERGE:  Debian packaging dependency rework

These commits rework (and simplify) Debian packaging dependencies again.  Problems addressed:
- Automatic install of kernel package dependencies from flavor module packages; **SEE BELOW**
- Fix substvars `unused/unknown substitution variable` warnings
- Clean up python deps
- Convert `preempt-rt` module package kernel deps from `substvars`
- Remove `debian/substvars`
- Migrate from deprecated `python-support` to `dh_python2`
See commit log for lots of detail.

@kinsamanka, please check out the top commit here for a proposal to make the same packaging work for your RPi kernels.  The idea is to add Debian-style (or otherwise distinct) `Provides:` relations to your kernel packages.  Also please review the second commit.