Merge pull request contiki-os#402 from adamdunkels/release-2-7

Bumped the version number from Contiki 2.6 to Contiki 2.7

Version 2.6

Version 2.6 release candidate 0

Update makefiles to git release tags and directories

Move 16-bit integer conversion operations into functions, as is done …

…already with 32-bit ops.

Update to Contiki version number 2.4

-> 2.3

Bumped version number to 2.2.3

Bumber version number to 2.2.2

2.2 -> 2.2.1