Fix rpl-udp build failure on native target, reported on contiki mailing list

http://sourceforge.net/mailarchive/forum.php?thread_name=a2b9cb98ac4a191867afc953113fabaa%40amnesiak.org&forum_name=contiki-developers
Merge pull request #408 from ADVANSEE/cc2538-uart

cc2538: lpm: Add registration mechanism for peripherals and use it
Merge pull request #425 from ADVANSEE/cc2538-gpio

cc2538: gpio: Bug fix and various improvements
Merge pull request #322 from ismb-pert/fix-rpd-udp

Fix build failure on rpl-udp reported on contiki mailing list
Merge branch 'master' into qidevices

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>