lowpan-tools: add git version

The 0.3 release was a year ago and git HEAD contains fixes to make it
work with contiki, so build from git instead of backporting all of the
patches.

Based on a patch by Koen Kooi <koen@dominion.thruhere.net>

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>