Add clean targets
Add clean targets

- Add several clean targets. For every project we create clean_all and
  ${CMAKE_PROJECT_NAME}-clean_all. Every exported target adds itself to
  these clean targets. We have two clean_all targets per project, to
  make the build output clearer, as the build output would otherwise
  repeatedly read "Built target clean_all". Having a distinguishable
  ${CMAKE_PROJECT_NAME}-clean_all target for each project makes each
  line of build output distinguishable as well.

falbrechtskirchinger/avr32-cmake-template,falbrechtskirchinger/avr32-cmake-template