tests: test_bloom: fix PROJDEPS concurrency

Our buildtests build one project concurrently for multiple boards.
The current creation of the `PROJDEPS` for `test_bloom` might fail if

* board `A` notices that the dependency is missing and gets preempted,
* board `B` creates the dependency and gets preempted,
* board `A` starts the creation but gets preempted in the middle of the process,
* board `B` works with a half complete created file.

This PR creates the dependency in the individual `BINDIR`.
Merge pull request #1338 from Kijewski/test_bloom-concurrency

tests: test_bloom: fix PROJDEPS concurrency
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