uart: Add primitive noise filtering on RX

A long cable attached to the UART can act as an antenna if disconnected
from the other end. This can cause noise on the receive side, possibly
as reflections from the transmit side, leading to an interrupt storm.
Filter this by adding a threshold of received characters without TX
ready, above which characters are dropped. This is disabled by default,
but has been tested with a threshold of 1000+. A high threshold is
recommended to avoid dropping characters during, for instance, a large
copy/paste from the other end.

Sponsored by: Juniper Networks, Inc.