Make `ARDMK_PATH` and `ARDMK_DIR` backward compatible

When support for ChipKit was added recently (issue #98) it broke the way
ARDMK_PATH and ARDMK_DIR were handled, if the package was installed
through package. It was only working if you have done a git checkout.

This commit fixes it, but searching for `Common.mk` both in
`ARDMK_DIR/arduino-mk` and `ARDMK_DIR/` directories.

Fix #110