Fix all warnings for native and qemu-i386

Missing returns, unused variables (only used for debugging), empty
translation units, missing function prototypes, and GNU extensions.
nativenet: change max packet size to 127

Real IEEE 802.15.4 transceivers have a payload of 127. Hence, for
testing - particular 6lowpan - it would make sense to have the same
limitation in nativenet. Especially to test fragmentation this is
helpful.
Merge pull request #1354 from OlegHahm/nativenet_packetsize

nativenet: change max packet size to 127
Merge pull request #1335 from Kijewski/fix-all-warnings-for-native-and-qemu-i386

Fix all warnings for native and qemu-i386
cpu - sam3x8e: fixed vtimer, added timer_set_absolute