dist/tools/jlink: refactor script

- Do not provide defaults for `${DBG}`, `${TUI}`, `${GDB_PORT}`,
  `${TELNET_PORT}` with special shell functions, but use canonical
  syntax for that
- Add `${DBG_EXTRA_FLAGS}` and pass them to GDB, as we do in OpenOCD
- Run `${DBG}` with `sh -c "..."` just like done in `openocd.sh` to
  allow passing flags to GDB in the same way independent of whether
  JLink or OpenOCD is used.

dist/tools/jlink/jlink.sh

@@ -68,10 +68,18 @@
 # The setsid command is needed so that Ctrl+C in GDB doesn't kill OpenOCD
 : ${SETSID:=setsid}
 
+# GDB command, usually a separate command for each platform (e.g. arm-none-eabi-gdb)
+: ${GDB:=gdb}
+# Debugger client command, can be used to wrap GDB in a front-end
+: ${DBG:=${GDB}}
+# Start with tui by default
+: ${TUI:=-tui}
+# Extra debugger flags, added by the user
+: ${DBG_EXTRA_FLAGS:=}
 # default GDB port
-_GDB_PORT=3333
+: ${GDB_PORT:=3333}
 # default telnet port
-_TELNET_PORT=4444
+: ${TELNET_PORT:=4444}
 # default J-Link command names, interface and speed
 _JLINK=JLinkExe
 _JLINK_SERVER=JLinkGDBServer
@@ -115,15 +123,6 @@ test_binfile() {
     fi
 }
 
-test_ports() {
-    if [ -z "${GDB_PORT}" ]; then
-        GDB_PORT=${_GDB_PORT}
-    fi
-    if [ -z "${TELNET_PORT}" ]; then
-        TELNET_PORT=${_TELNET_PORT}
-    fi
-}
-
 test_elffile() {
     if [ ! -f "${ELFFILE}" ]; then
         echo "Error: Unable to locate ELFFILE"
@@ -132,12 +131,6 @@ test_elffile() {
     fi
 }
 
-test_tui() {
-    if [ -n "${TUI}" ]; then
-        TUI=-tui
-    fi
-}
-
 test_serial() {
     if [ -n "${JLINK_SERIAL}" ]; then
         JLINK_SERIAL_SERVER="-select usb='${JLINK_SERIAL}'"
@@ -145,12 +138,6 @@ test_serial() {
     fi
 }
 
-test_dbg() {
-    if [ -z "${DBG}" ]; then
-        DBG="${GDB}"
-    fi
-}
-
 test_term() {
     if [ -z "${JLINK_TERMPROG}" ]; then
         JLINK_TERMPROG="${_JLINK_TERMPROG}"
@@ -246,9 +233,6 @@ do_debug() {
     test_serial
     test_version
     test_elffile
-    test_ports
-    test_tui
-    test_dbg
     # start the J-Link GDB server
     ${SETSID} sh -c "${JLINK_SERVER} ${JLINK_SERIAL_SERVER} \
                            -nogui \
@@ -261,7 +245,7 @@ do_debug() {
     # save PID for terminating the server afterwards
     DBG_PID=$!
     # connect to the GDB server
-    ${DBG} -q ${TUI} -ex "tar ext :${GDB_PORT}" ${ELFFILE}
+    sh -c "${DBG} -q ${TUI} -ex \"tar ext :${GDB_PORT}\" ${DBG_EXTRA_FLAGS} \"${ELFFILE}\""
     # clean up
     kill ${DBG_PID}
 }
@@ -270,7 +254,6 @@ do_debugserver() {
     test_config
     test_serial
     test_version
-    test_ports
     # start the J-Link GDB server
     sh -c "${JLINK_SERVER} ${JLINK_SERIAL_SERVER} \
                            -nogui \