Moved Common.mk functions back into Arduino.mk which fixes AUTO_ARDUINO_...

...DIR calculation, which fixes issue #147 allowing much simpler user Makefiles e.g.

 BOARD_TAG = atmega328bb
 ISP_PROG = usbasp
 include /usr/share/arduino/Arduino.mk

Fixed ARDMK_FILE arduino.mk capitalisation

I know it was nice to externalise some common functions but there weren't many and one at least wasn't working and required workarounds in the main Arduino.mk