Packaging tweaks:  feature set rules hooks and auto-'Provides:'

Enable a feature set to include its own makefile in
debian/rules.featureset-<featureset>

Automatically add "Provides: linux-image-<featureset>" and "Provides:
linux-headers-<featureset>" in feature set packages
Add Xenomai feature set

- README file with build instructions
- Add 'xenomai' to config/*/defines
- Add xenomai kernel config
- Featureset rules, debian/rules.featureset-xenomai:
  - add Xenomai patch as prereq of debian/control
- Add Build-Depends: linux-patch-xenomai and me as Maintainer: