uart: Add [rx|tx]_packet_delimiter options.

This is a convenience feature that emits summary annotations ("packets")
that comprise all data values that were decoded until a specified delimiter
value is seen (as last data value of the "packet").

Example use-cases include ASCII data where it can be convenient to
"packetize" whenever a 10/0x0A value (newline) is seen, or some
protocols which have a fixed "marker" value (e.g. 0x55) as last
value in the "packet".

The annotations are affected by the selected 'format' option, i.e. the
user can get summaries in ASCII or hex or other formats.

This is just an (intentionally very simple) helper/convenience improvement
and is NOT meant to replace "proper" stacked decoders for UART-based protocols.