sync_module: split SyncMsg and use max pkt size

This is a first version. This avoid "insufficent bytes"' errors due
to the pack with a max size of 8k (which can be the case)
In a future version, the SyncMsg will be splitted in partial data
to only use the necessary size and could take any wanted size.

Change-Id: Ic4b86a8ac8c3c49063e5650fefbb1f969b458db6