anytransceiver pseudomodule

Adds a new pseudo-module named `defaulttransceiver` with accompanying macro `TRANSCEIVER_DEFAULT`.

The board Makefiles include their respective default transceiver module when `defaulttransceiver` should be built.
The `defaulttransceiver` module itself is never built.
The `TRANSCEIVER_DEFAULT` macro is set in `transceiver.h`.

Applications can use the `TRANSCEIVER_DEFAULT` macro to communicate with the transceiver thread if they do not care what radio they use.
The application Makefile does not need to know which module is actually present anymore.

Additionally a `PSEUDOMODULES` variable is introduced to the make system in `Makefile.pseudomodules` that is used to filter out modules that do not result in a binary of the same name but can be used to build other modules as a dependency.

Implements https://github.com/RIOT-OS/RIOT/issues/432

I deleted all comments as they were outdated (either addressed or explained in this very comment you are reading now). Tabula rasa! (2014-02-11 20:26 Europe/Berlin)