iot-devkit.inc: update supported distros list
iot-devkit.inc: update supported distros list

* Remove old Debian versions
* Add Fedora 20 (the current release)

Signed-off-by: Paul Eggleton <737d4dae46a541673d7bf4e0d5b32714b16528d8@linux.intel.com>

MCherifiOSS/meta-intel-iot-devkit