Adding support for SDK 1.5

Hi,
 I'm writing a atmega2560 and I found this beatiful toolchain. I've found some little issue using Arduino 1.5 and with some library. So I change Arduino.cmake file for
* If SDK >= 1.5 register hardware for avr or ${ARDUINO_PLATFORM} hardware (default AVR)
* include libraries from arduino hardware platform directory
* handle board mega for mcu property in boards.txt using ARDUINO_CPU variabile to select specific cpu menù
* log all libraries and dependencies for libraries
* add include for ${LIBRARY}/src directory for Arduino libs with sources in src/
* set AVROBJCOPY in cache, because everytime I change CMakeLists.txt I loose value!

Hope it is helpful for you.