set received parent lifetime when creating a new parent
initializes RPL instances
printf is used in sixlowpan mac layer, stack needs to be adjusted
use local packet_length variable in sendto

Having a global packet_length variable doesn't seem to be the best
idea. To avoid trouble, a scope local variable should be preferred.
check if preferred parent was set before comparing
Merge pull request #384 from OlegHahm/sixlowpan_fixes

printf is used in sixlowpan mac layer, stack needs to be adjusted
enforce KERNEL_CONF_STACKSIZE_PRINTF in kernel.h
Merge pull request #397 from thomaseichinger/kernel_conf_stacksize_printf

enforce KERNEL_CONF_STACKSIZE_PRINTF in kernel.h
Merge pull request #383 from OlegHahm/rpl_fixes

Small RPL fixes