splitted up Makefiles into subdirectories

- custom modules can now be included in one of the
  custom.mk files

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
added custom.mk to .gitignore

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
Merge pull request #258 from mguentner/makesystem-rebase

Makesystem: Splitted up Makefiles into subdirectories