make: refactor make system (parallelism, deduplication)

Almost everything was build sequentially in RIOT, because we employed
explicit for-loops to build directories (DIRS). This PR makes our make
system use normal dependencies to build directories.

All our compiling rules were duplicated, once for the application, once
for modules. This PR makes the application a normal module, removing
this duplication.
Make uses a "jobserver" to have the number of instances running that you specified at the original invocation. Otherwise you had N*N*… running instances.