#30 Simple callback for session based ciphers selection

[Achim Kraus]

According the LWM2M technical specification a client should be able to select the cipher based on its configuration (LWM2M security object). So wakaama (LWM2M C implementation) would benefit from a session based cipers selection.

Ticket 22 already introduce such a session based cipher selection but comes with a more related stuff.

The impact of this patch on the code is much smaller. It delegates the selection mainly to a callback similar to the psk- or ecc-key management.

The patch is based on the current "develop" branch
(commit 1719a06e3db6c9667bdaf848c8b03b81dc029de3).