Update platform.txt

This copies platform.txt from Arduino git master verbatim, except the
name and version are changed.

Compared to the previous platform.txt, this adds -DARDUINO_{build.board}
and -DARDUINO_ARCH_{build.arch} and adds a recipe for compiling .S
files.
Add build.board property

This causes the ARDUINO_PINOCCIO macro to be defined when the Pinoccio
boards are selected. This does not distinguish between the 256RFR2 and
128RFR2 versions, but since the later was only a prototype, and the
regular CPU defines are also available, this shouldn't be a problem (the
alternative would be to check for both boards everywhere in the code,
which isn't nice either).
Use the regular arduino core

Previously, the pinoccio boards used an older copy of the regular
arduino core that is included in this repository. By explicitely listing
the core as "arduino:arduino" (instead of the previous implicit
pinoccio:arduino"), it uses the arduino core from the hardware/arduino
folder, i.e., the regular arduino core.

The variant folder is still taken ofr the hardware/pinoccio/variants
folder as before.
Define __PROG_TYPES_COMPAT__

The regular arduino core uses some types like prog_char, which are
deprecated on newer avr-libc versions. Since the Pinoccio requires a
newer toolchain than is included with Arduino, this breaks.

A fix is pending in the Arduino repo, but until it is in a released IDE
version, this workaround should fix things. See also
https://github.com/arduino/Arduino/pull/1695
Remove cores

Now that we're using the official Arduino core again, there is no point
in carrying around two more copies of the core.
Turn pin definitions into macros

This allows using them in #if. See also
https://github.com/arduino/Arduino/issues/758