ROMFS: ignore comments and newlines in startup files, text in mixer files

This adds a Python script  that prunes the mixer and startup files.
However, the impact on the px4fmu-v1_default.bin file is not that huge:

1008488 - 986984 = 21504 bytes

Compilation works on Arch Linux using Python 2.7 and 3.3.

This needs a short bench test on  FW and MC.