net: Initial import of a global packet buffer

Cleaner alternative to #1129. Queue in #1640 

This is basically a static malloc + sugar
```diff
- * @brief   Copies packet data into the packet buffer, savely.
+ * @brief   Copies packet data into the packet buffer, safely.
  *
- * @detail  This is to be prefered to memcpy, since it is thread-save and
+ * @detail  This is to be preferred to memcpy, since it is thread-safe and
```