net: Initial import of a global packet buffer

Cleaner alternative to #1129. Queue in #1640 

This is basically a static malloc + sugar
```
- * @detail  The rational here is to have at least space for 4 full-MTU IPv6
- *          packages (2 for incoming, 2 for outgoing; 2 * 2 * 1280 B = 5 KiB)
+ * @detail  The rational here is to have space for at least 4 full-MTU IPv6
+ *          packages (2 incoming, 2 outgoing; 2 * 2 * 1280 B = 5 KiB)
```