dist/tools/jlink: send output to /dev/null

While running `make term` JLinkExe is expecting commands and thus
can be disturbed by other JLink commands, e.g. `make reset`.
This enable `make test` (which runs those two commands at the same
time) on target using JLinkExe as a programmer/debugger.

dist/tools/jlink/jlink.sh

@@ -245,7 +245,7 @@ do_term() {
             -speed '${JLINK_SPEED}' \
             -if '${JLINK_IF}' \
             -jtagconf -1,-1 \
-            -commandfile '${RIOTTOOLS}/jlink/term.seg' & \
+            -commandfile '${RIOTTOOLS}/jlink/term.seg' >/dev/null & \
             echo  \$! > $JLINK_PIDFILE" &
 
     sh -c "${JLINK_TERMPROG} ${JLINK_TERMFLAGS}"