ContikiMAC fix

To fix #297, I introduced the "ContikiMAC framer" in #557. It handles the creation and parsing of the ContikiMAC header and can be enabled as follows:
```c
#undef NETSTACK_CONF_FRAMER
#define NETSTACK_CONF_FRAMER contikimac_framer
```
The "decorated" framer defaults to `framer_802154`. If needed, this can be changed like that:
```c
#undef CONTIKIMAC_FRAMER_CONF_DECORATED_FRAMER
#define CONTIKIMAC_FRAMER_CONF_DECORATED_FRAMER decorated_framer
```
Unfortunately, the current ContikiMAC framer lacks a `length` function. This pull request fixes this.