ContikiMAC framer fix

[kkrentz]

The contikimac_framer lacks a length function, which is why it does not currently work.

[nvt]

👍

This framer is not set anywhere in the source tree, and it is probably not clear to many users when it is needed. Perhaps it should be made the default for those platforms and examples that use Rime over ContikiMAC, since it is often the case that Rime packets are too short for ContikiMAC's strobing mechanism to catch them reliably.

[kkrentz]

You're right. I've added a note to the wiki page https://github.com/contiki-os/contiki/wiki/Contikimac. Also, I'm now defaulting to this framer when using rime over ContikiMAC. The contikimac_framer became necessary to handle the interplay with the LLSEC layer. Zeroes have to be appended after securing a frame and removed before unsecuring a frame.

[nvt]

Looks good, but you should probably retain the copyright year of the original ContikiMAC source code in the ContikiMAC framer.

[kkrentz]

Done

[nvt]

This setting belongs in the section named "Network Stack Configuration", where the Rime driver is set.

[kkrentz]

Now the positions of the framer definitions are being preserved.