Make: Use generic app descriptior for mac spotlight search

kMDItemKinde is a localized string, only in EN OSX versions the value
is Application, changed to generic ContentType metafild
mk/find_tools.mk: add AVRSIZE variable
mk/board_avr.mk: add avr-size output
mk: add c++ warnings

https://groups.google.com/forum/#!searchin/drones-discuss/warnings/drones-discuss/oRG8226F7_k/98dOJlW-DwAJ
mk: added macports g++ -compiler support for MAC OS X

- macports' version of avr-size doesn't support certain flags, so we
added a fallback on the bundled Arduino version
mk/board_avr: added output of used compiler

this is especially useful on OSX, to check if the macports or arduino
gcc was used