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