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