Update toolchain to Atmel 3.5.2

[cmaglie]

Sources: http://distribute.atmel.no/tools/opensource/Atmel-AVR-GNU-Toolchain

Changeset:

• the build for avrdude has been moved here: https://github.com/arduino/avrdude-build-script
• the build script has been unified and slightly simplified
• use directly Atmel' source code instead of patching upstream versions