Enable C99, with gnu extensions

The default for GCC is gnu90 (C90 with gnu extensions), which does not
allow things like declaring variables in for loops, or // comments. This
switches to gnu99, which allows compiling more code.
Explicitely specify std=gnu++98 for C++ compilation

This is the default for GCC, but specifying it explicitely makes it more
clear what C standard is being used.
Remove atmega16u2 code and toolchain bits

This repository should contain just the Pinoccio core for use with the
Arduino IDE, so these bits will be moved to another place.
Move the hardware/pinoccio/avr directory to top level

This allows cloning this repository into the the `hardware` directory in
the Arduino sketchbook to make it work, instead of having to clone it
separately and copy or symlink it into the `hardware` directory.
Update platform.txt for arduino-avr-toolchain-nightly-gcc-4.8.1

This updates the platform.txt with the latest version from the
experimental Arduino IDE version with updated toolchain
(ide-1.5.x-avr-toolchain-gcc-4.8.1 branch):

https://github.com/arduino/Arduino/blob/a111f824c9dd5aa5579b64acfb0f81f157ac3658/hardware/arduino/avr/platform.txt

Apart from some cosmetical changes, this makes uploading work with the
IDE built from that branch. However, it also breaks uploading on Linux
with the regular IDE.
Delete README.md

The info in it is now moved to the firmware-pinoccio repository, so no
longer needed here.
Remove __PROG_TYPES_COMPAT__ workaround

In Arduino 1.5.6, https://github.com/arduino/Arduino/pull/1695 got
merged which removes the need for this workaround.
Merge the 128rfa1 and 256rfr2 board.txt entries

Now, there is just a single "Pinoccio Scout" board in the Arduino IDE.
However, there is now also a "Processor" submenu, that allows choosing
between the two boards.