Audio modifications,
Audio improvements,

Audio could not access anything in hsync irq, was put in normal thread.
  also introduced double buffering. The real fix would be to put it in a
  real double buffer DMA to Audio DACs.

bitbox.h : modified message(...) to behave like printf (also in emu)
bitbox.mk : parametrized start flash so that make stflash works with a bootloader
blitter : improved a little 16x16 blitter doc in .h
emulator : implemented chdir.
ffconf.h : was too restrictive (also meddled with interger types)