Provide openocd_cmd and add some documentation (#492)

* Provide openocd_cmd

* Add openocd instructions for dongle

deploy.py

@@ -270,7 +270,7 @@ class OpenSKInstaller:
         jlink_speed=1200,
         openocd=self.args.programmer == "openocd",
         openocd_board=board.openocd_board,
-        openocd_cmd="openocd",
+        openocd_cmd=self.args.openocd_cmd,
         openocd_commands=copy.copy(board.openocd_commands),
         openocd_options=copy.copy(board.openocd_options),
         jtag=False,
@@ -1005,6 +1005,14 @@ if __name__ == "__main__":
       help=("Sets the method to be used to flash Tock OS or the application "
             "on the target board."),
   )
+  main_parser.add_argument(
+      "--openocd_cmd",
+      dest="openocd_cmd",
+      metavar="CMD",
+      default="openocd",
+      help=("Specifies a custom command to use when calling openocd. Can be "
+            "used to pass arguments i.e. 'openocd -s /tmp/openocd_scripts'."),
+  )
 
   main_parser.add_argument(
       "--no-tockos",

docs/boards/nrf52840_dongle.md

@@ -59,14 +59,48 @@ Follow these steps:
 
     ![Nordic dongle retainer clip](../img/dongle_clip.jpg)
 
-1.  Depending on the programmer you're using, you may have to adapt the next
-    command line. Run our script for compiling/flashing Tock OS on your device:
+#### JLink
 
+Run our script for compiling/flashing Tock OS on your device:
+
+```shell
+$ ./deploy.py --board=nrf52840_dongle --programmer=jlink
+```
+
+#### OpenOCD
+
+1.  Create your openocd config, named `nordic_nrf52840_dongle.cfg` in the
+    appropriate location:
     ```shell
-    $ ./deploy.py --board=nrf52840_dongle_opensk --programmer=jlink
+    mkdir -p ${HOME}/.openocd/board
+    touch ${HOME}/.openocd/board/nordic_nrf52840_dongle.cfg
+    ```
+    
+    Paste the following st-link example and edit the specific setup to your needs:
+    ```
+    # Specific setup
+    source [find interface/stlink-dap.cfg]
+    transport select dapdirect_swd
+    
+    # The rest should be kept the same
+    set CHIPNAME nrf52840
+    source [find target/nrf52.cfg]
     ```
 
-1.  Remove the programming cable and the USB-A extension cable.
+1.  Test your config:
+       
+    ```shell
+    openocd -f board/nordic_nrf52840_dongle.cfg
+    ``` 
+
+1.  Run the deploy script with the appropriate options, i.e.:
+
+    ```shell
+    ./deploy.py --board=nrf52840_dongle --opensk --programmer=openocd
+    ```
+
+
+Finally, remove the programming cable and the USB-A extension cable.
 
 ### Buttons and LEDs