make: s/PROJDEPS/APPDEPS/

fixes https://github.com/RIOT-OS/RIOT/issues/1284
doc: improve APPDEPS example

Addresses https://github.com/RIOT-OS/RIOT/pull/1339#discussion_r14050325
make: evaluate the Git SHA only once
Merge pull request #1339 from LudwigOrtmann/issue-1284

make: s/PROJDEPS/APPDEPS/
Merge pull request #1340 from Kijewski/evaluate-git-sha-once

make: evaluate the Git SHA only once
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: make DIRS usable for applications
make: create dependencies as side effect
make: use abspath for better error messages
make: ensure order of `$(AR)` calls