uni-t-ut32x: improve robustness of packet parser, more diagnostics

uni-t-ut32x: improve robustness of packet parser, more diagnostics

Always print the data bytes of received buffers in the packet parser,
then check some more fixed fields to not process invalid packets, then
process the packet content as the previous implementation did.

Call the packet parser for incomplete packets and discarded input
buffers as well (initial synchronization, re-sync after comm errors).
This results in the availability of more diagnostics during development.

Pass the packet's location and size from outside. This prepares the
logic to cope with situations where the receive buffer contains multiple
(potentially incomplete) packets.