ext: Import Segger J-Link RTT library

The Real Time Terminal functionality of the Segger J-Link
debug probe and software suite allows applications to write
to a certain RAM area that is read in real-time by the
debugger. This is specially useful when a UART is not
available to provide a console.

Origin: Segger J-Link 6.10m
URL: https://www.segger.com/downloads/jlink


Maintained-by: External

Change-Id: I47f5e2d5172797004a3f34ea2f002cd781a22cc8
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>