shell: rtt: Add detection of host presence

If host is not reading RTT data (because there is no PC connection
or RTT reading application is not running on the host), thread will
stuck continuously trying to write to RTT. All threads with equal or
lower priority are blocked then. Adding detection of that case and
if host is not reading data for configurable period then data is
dropped until host accepts new data.

Similar solution is using in RTT logging backend.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>