Read Arduino.mk.local if it exists.

If you don't want to clobber your environment, don't want to mess with
the upstream makefile and dont want to tweak every project makefile, you
can now place an Arduino.mk.local file with your settings into the same
directory as Arduino.mk. If this new file exists, it will be included.

Signed-off-by: Ralf Doering <ralf@rdoering.net>