New AVR toolchain

This PR extends #1806 replacing the old toolchain with the latest version available from atmel [0]
It includes binutils 2.23.2, gcc 4.8.1, avr-libc 1.8.0, avrdude 6.0.1

New toolchain is built with the scripts available at [1]

[0] http://distribute.atmel.no/tools/opensource/Atmel-AVR-GNU-Toolchain/3.4.3/
[1] https://github.com/arduino/toolchain-avr/tree/atmel-3.4.3