scripts/logging: Add support to RTT reading from JLink

Update the live logger to be able to read the logs from JLink, over the RTT
protocol.
The support for the jlink-rtt mode is done similarly to the currently
existing support for files and serials.

The implementation for opening the jlink for reading was inspired by the
available example from the pylink python implementation here:
https://github.com/square/pylink/blob/master/examples/pylink-rtt



In contrast to the other readers, the jlink-rtt does not support being
polled from 'select'.
To make it possible to work with such readers, the code was updated to
allow for busy-waits, instead of smart polling.
This change does not affect the current readers, but support the new
reader, and allows for future readers which does not support smart polling.

Signed-off-by: Omri Sarig <omsi@demant.com>