boards/slstk3401a: Add reset before flashing
boards/slstk3401a: Add reset before flashing

It seems that this board needs a reset command before flashing with jlink
This adds the reset - r - to the JLINK_PRE_FLASH

authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,basilfx/RIOT,OTAkeys/RIOT,josephnoir/RIOT,aeneby/RIOT,josephnoir/RIOT,rfuentess/RIOT,smlng/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,smlng/RIOT,yogo1212/RIOT,kYc0o/RIOT,OTAkeys/RIOT,x3ro/RIOT,OTAkeys/RIOT,authmillenon/RIOT,rfuentess/RIOT,kYc0o/RIOT,cladmi/RIOT,x3ro/RIOT,yogo1212/RIOT,x3ro/RIOT,authmillenon/RIOT,mtausig/RIOT,x3ro/RIOT,toonst/RIOT,authmillenon/RIOT,mtausig/RIOT,rfuentess/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,basilfx/RIOT,mtausig/RIOT,OlegHahm/RIOT,smlng/RIOT,kYc0o/RIOT,josephnoir/RIOT,yogo1212/RIOT,mtausig/RIOT,jasonatran/RIOT,cladmi/RIOT,yogo1212/RIOT,ant9000/RIOT,josephnoir/RIOT,cladmi/RIOT,toonst/RIOT,ant9000/RIOT,smlng/RIOT,aeneby/RIOT,ant9000/RIOT,miri64/RIOT,toonst/RIOT,jasonatran/RIOT,miri64/RIOT,OTAkeys/RIOT,basilfx/RIOT,kYc0o/RIOT,josephnoir/RIOT,miri64/RIOT,jasonatran/RIOT,toonst/RIOT,basilfx/RIOT,basilfx/RIOT,jasonatran/RIOT,OlegHahm/RIOT,mtausig/RIOT,aeneby/RIOT,ant9000/RIOT,OlegHahm/RIOT,cladmi/RIOT,OlegHahm/RIOT,kaspar030/RIOT,aeneby/RIOT,smlng/RIOT,rfuentess/RIOT,yogo1212/RIOT,kaspar030/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,cladmi/RIOT,toonst/RIOT,x3ro/RIOT,yogo1212/RIOT,rfuentess/RIOT,aeneby/RIOT,kaspar030/RIOT,RIOT-OS/RIOT