tools/openocd: correctly check reset_config variable value
tools/openocd: correctly check reset_config variable value

This avoids the following warning message when running OpenOCD: 'openocd.sh: line 124: [: : integer expression expected'

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