Configurability updates

This patch adds four configurability options:

* The addition of a PLATFORMAPPS Makefile variable, which allows platform Makefiles to add APPS to be compiled into the firmware. This is useful for placing code that is platform agnostic, like driver or low-level protocols, into an apps/ directory and then including it in the platforms that need it.
* ContikiMAC, Telnetd, and RIme collect options made configurable with `contiki-conf.h` options instead of needing to edit the source code files.