work:
- configured FatFs to support long file names
- configured FreeRTOS for max sleep mode = EM1 (this makes the GPS work with the
  OS putting the MCU in sleep mode)
- set optimization level to -O0, to make the accelerometer and compass work
- fooling around in GPS background task to try to store GPS trace

The bloody compiler optimization was screwing over NMEA parsing, which made the
GPS not work anymore.

Thank you GCC for taking away 2 hrs of my life, just like that: POOF!

For now, I was only testing writing a file to the SD card from the FreeRTOS
apps. It doesn't yet work.

All other apps use the same button to exit, so the USB mass storage app was
changed in kind.

The setting_apply() and setting_get() functions were added, to make it easier to
work with the settings structs. Also, to make the GMT offset setting work with
these two functions, the setting was split into two settings, now of type
setting_t.