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
Merge pull request #1339 from LudwigOrtmann/issue-1284

make: s/PROJDEPS/APPDEPS/