dist/jlink: do not show the GUI.
dist/jlink: do not show the GUI.

Since J-Link V6.74, a GUI is shown when flashing from the command line.
This steals the focus of the terminal. Adding `-nogui 1` solves the
issue, but raises the minimum supported version to V6.74 (released
around June 2020).

Note that JLinkGDBServer has a slightly different `-nogui` option.

authmillenon/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT