west: runner: add support for NXP's linkserver

Linkserver is a utility for launching and managing GDB servers for NXP
debug probes, which also provides a command-line target flash programming
capabilities. Linkserver can be used with NXP MCUXpresso for Visual Studio
Code.

For more information about LinkServer, please visit the LinkServer web
page (link [1] below).

This commit adds a runner to west, supporting debug and flash commands.

Documentation is also added.

[1] - LinkServer web page:
    https://www.nxp.com/design/software/development-software/mcuxpresso-software-and-tools-/linkserver-for-microcontrollers:LINKERSERVER



Signed-off-by: Yves Vandervennet <yves.vandervennet@nxp.com>