asterisk13: change some default file locations

- Set data dir to /usr/share/asterisk and use default locations for
  everything else.
- This is more in line with FHS and fixes issue #38. Database files are
  now written to tmpfs (/var is mounted there). This prevents the
  rootfs from filling up. It also prevents the flash to degrade during
  constant database rewriting.
- As the data dir changes, so does the location of the sound files. This
  commit also takes care of that.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>