reworked the main loop #11

With the latest code cleanup, issues were introduced regarding the mounting and
unmounting of the filesystem.

• Sometimes the board would wake up from sleep and perform mounting and
  unmounting of the filesystem in a more or less short loop before going
  back to sleep. I suspect this might be due to the sleep getting
  interrupted without any button press, aka every 10 minutes or so.
• This code also created a bug where it would create then mount a new
  filesystem on the flash after erasing it, breaking the purpose for that
  function in the first place which is to erase the flash and nothing
  else.
• It generally does not make sense to unmount a filesystem right after having
  mounted it.

Main is now calling mountFilesystem and unmountFilesystem where
appropriate:

• Before running doUSBMSD, main unmounts the filesystem. When doUSBMSD
  is done, main once again mounts the filesystem.
• Before erasing the flash with doERASE, main unmounts the filesystem,
  and never mounts it again.
• As a tidying up step, the filesystem gets unmounted, same as the
  button thread terminating.

reworked the main loop