Ide 1.5.x platform.local.txt

This allows overriding values from `platform.txt` using a `platform.local.txt`.

Additionally, the third commit makes the avr platform.txt not rely on a `compiler.path` value set by the IDE, see the commit for the rationale.

As discussed at https://groups.google.com/a/arduino.cc/d/msg/developers/4jL4nL_YJ1k/E-AW4XFD-W0J