If the attribute name is passed, then it is the one used to process the option
--target[s] of waf build. The board name should be used only in configuration
time. The build targets should be board/platform agnostic.
Now, instead of using `waf --target ArduCopter.sitl`, we use `waf --target
ArduCopter` and the binary continues named as "ArduCopter.sitl".
It was implemented in such a way that gtest is required only if the user wants
to build and run tests. Initially we're considering all tests should be gtests.
We can change that assumption in the future if necessary.
We're currently using the tests standard error for reporting tests. We can add
TAP later to integrate with other tools.
Additionally, this patch simplifies the exclude patterns passed to
collect_dirs_to_recurse.
Gustavo Jose de Sousa
Caio Marcelo de Oliveira Filho