Override complete compiler tool paths for chipKIT.

Previously we were setting the AVR tools path to the PIC32 tools path because it
made grabbing the compiling tools easier. Consequently, it made finding the
avrdude path much harder, especially since the avrdude files are in different
locations in the Linux distributions of MPIDE and Arduino.

Instead, we set the AVR tools path to the *correct* path (where *AVR* dude
lives), and totally override the CC, CXX, etc. paths to point to their PIC32
equivalents.
Merge pull request #159 from peplin/simpler-crossplatform-paths

Override complete compiler tool paths for chipKIT.