add mising config for `openocd_target`

boards/nano-g2-ultra.json

@@ -46,7 +46,8 @@
     ],
     "debug": {
         "jlink_device": "nRF52840_xxAA",
-        "svd_path": "nrf52840.svd"
+        "svd_path": "nrf52840.svd",
+        "openocd_target": "nrf52.cfg"
     },
     "frameworks": [
         "arduino"
@@ -69,4 +70,4 @@
     },
     "url": "https://wiki.uniteng.com/en/meshtastic/nano-g2-ultra",
     "vendor": "BQ Consulting"
-}
+}