envsetup: new function cmka

- The build system adds a clean-<module> target
  for each module / package.
- This utilizes that and cleans and builds using mka.
- If no argument is supplied, it runs "mka clean".
- If argument is (bacon|otapackage|systemimage), it
  runs installclean.

Change-Id: I582ac2f92df3278c41c254c73e9c02ad7f406851
(cherry picked from commit eb74c2f2b9bb708e77edfdbb8ba0d0b716fef2fc)
(Also sneak in another minor change, a help entry for mkap)