Improvements to the core/net/ code

This patch includes four improvements to the `core/net` code:

* A new mechanism for dynamically computing the maximum packet size of 6lowpan packets. This was previously hard-coded, which meant that we could not take advantage of additional space in the header caused by more efficient MAC layer headers, and that there were corner cases where the packet could become too large (so we had to be a bit too conservative when hard-coding the maximum packet size).
* Change to the 802154 framer functions that is needed by the 6lowpan header size mechanism.
* Allow nullrdc to send link-layer acks, and make the time spent waiting for incoming ACKs configurable.
* Rime collect: send announcements only if a node is a router. This change makes it easier to have larger networks where only a few nodes are routers.