Exclude *.c files from make doc
Introduce macro to define files that should be deleted on make clean
Move .elf file to it's own target
Make directory creation in object compilation more makey
Merge pull request #1137 from authmillenon/no_c_doc

doc: Exclude *.c files from make doc
Merge pull request #1098 from authmillenon/make-addon

make: Make make more flexible
make: remove clutter

Remove some outdated example code, and simplify output folder creation.
make: parallelize creation of board and base code
 make: add `BOARD_INSUFFICIENT_RAM` blacklisting

Currently most blacklistings for examples and tests are done because the
board provides too little RAM or ROM. Besides of the actual linking all
the compiling should nevertheless work just fine.

This PR adds the variable `BOARD_INSUFFICIENT_RAM` to tell the
`buildtest` to compile the code for a board, but omit the linking step.
Use `BOARD_INSUFFICIENT_RAM` in examples and tests