The Make Build System
RIOT uses a recursive Makefile system. The starting point for these structure is typically the Makefile within the application's Makefile. Among other variables it sets the path to the RIOT repository as $RIOTBASE.
Usually, the last thing in an application's Makefile is to include the Makefile.include from $RIOTBASE.
In the simplest case a module just needs to provide a Makefile that does nothing more than including Makefile.base from $RIOTBASE. If the name of your module does not match the name of the directory containing the code, one has to set the variable MODULE as well. E.g. by adding MODULE = FOOBAR as the first line of the module's Makefile.
All RIOT specific variables used in the Make build system are listed and briefly explained in Makefile.vars.