In the previous version the decoder could in rare cases get synced from
the middle of data stream in case of data byte equal to the S.Bus start
of frame (SOF) byte (wrong data will be rejected but it was not perfect).
Now it waits for the real start of frame and then checks the SOF byte.
All receivers now fall under the same driver API provided
by pios_rcvr.c.
This is part of a larger sequence of commits that will
switch the receiver selection over to boot time dynamic
configuration via UAVObjects.
