A warning concerning the crossdev section: The current version of crossdev, 0.9.17 (binary is called crossdev-stable!) doesn't want to compile avr-g++ at all. I don't know why, but for avr, crossdev is set to always emerge to a stage without g++ which arduino needs. Comment out STAGE= in /usr/bin/crossdev-stable line 106 and you should hopefully be fine. YMMV.