dist/tools/jlink: remove setsid for launching JLinkExe for term

When executing `make test` on devices using JLink, testrunner
launches `make term` which calls `jlink.sh term_rtt`. When finished
the father process is killed but `setsid` has launched JLinkExe
as another subprocess, which is not killed by `os.killpg` from
testrunner since it doesn't belong to the same group.

dist/tools/jlink/jlink.sh

@@ -240,7 +240,7 @@ do_term() {
     # don't trapon Ctrl+C, because JLink keeps running
     trap '' INT
     # start Jlink as RTT server
-    setsid sh -c "${JLINK} ${JLINK_SERIAL} \
+    sh -c "${JLINK} ${JLINK_SERIAL} \
             -device '${JLINK_DEVICE}' \
             -speed '${JLINK_SPEED}' \
             -if '${JLINK_IF}' \