Add jlink_serial_no argument to specify target

-adds flag `--jlink-serial-no XXXX` to specify target board

README.md

@@ -287,6 +287,8 @@ operation based on the requirements of a particular hardware platform.
   bundle using only a single flash command. This will require that anytime any
   app changes in any way (e.g. its header changes or the app is updated or a new
   app is installed) all apps are re-written.
+- `--jlink-serial-no`: This selects which attached JLink device Tockloader is to
+  interact with using the specified JLink serial number.
 
 Credentials and Integrity Support
 ---------------------------------

tockloader/jlinkexe.py

@@ -35,6 +35,9 @@ def __init__(self, args):
             self.jlink_cmd = "JLinkExe"
             if platform.system() == "Windows":
                 self.jlink_cmd = "JLink"
+
+        # Obtain serial number if --jlink_serial_no argument provided
+        self.jlink_serial_no = getattr(self.args, "jlink_serial_no")
 
         # By default we assume that jlinkexe can be used to read any address on
         # this board, so we set `address_maximum` to None. In some cases,
@@ -195,6 +198,10 @@ def _run_jtag_commands(self, commands, binary, write=True):
                 self.jlink_speed,
                 jlink_file.name,
             )
+
+            # append target selector if serial number provided
+            if self.jlink_serial_no:
+                jlink_command += " -USB {} ".format(self.jlink_serial_no)
 
             logging.debug('Running "{}".'.format(jlink_command))
 
@@ -456,10 +463,11 @@ def run_terminal(self):
             return
 
         logging.status("Starting JLinkExe JTAG connection.")
-        jtag_p = subprocess.Popen(
-            "{} -device {} -if {} -speed {} -autoconnect 1 -jtagconf -1,-1".format(
-                self.jlink_cmd, self.jlink_device, self.jlink_if, self.jlink_speed
+        command_str = "{} -device {} -if {} -speed {} -autoconnect 1 -USB {} --jtagconf -1,-1".format(
+                self.jlink_cmd, self.jlink_device, self.jlink_if, self.jlink_speed, 
+                "" if self.jlink_serial_no is None else self.jlink_serial_no
             ).split(),
+        jtag_p = subprocess.Popen(
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
         )

tockloader/main.py

@@ -778,6 +778,11 @@ def main():
     parent_channel.add_argument(
         "--jlink-if", help="The interface type to pass to JLinkExe."
     )
+    parent_channel.add_argument(
+        "--jlink-serial-no", 
+        default=None,
+        help="Specify a specific JLink via serial number. Useful when multiple JLinks connected to same machine."
+    )
     parent_channel.add_argument(
         "--openocd-board", help="The cfg file in OpenOCD `board` folder."
     )