vpnkit: Add a config to specify the socket

boards/sim/sim/sim/NETWORK-VPNKIT.txt

@@ -11,6 +11,7 @@ Configuration
     CONFIG_SIM_NETDEV=y
     CONFIG_SIM_NETDEV_TAP is not set
     CONFIG_SIM_NETDEV_VPNKIT=y
+    CONFIG_SIM_NETDEV_VPNKIT_PATH="/tmp/vpnkit-nuttx"
 
 You can use the sim:vpnkit configuration, which includes the above
 settings.
@@ -35,5 +36,5 @@ You can use it as the following:
     % vpnkit --ethernet /tmp/vpnkit-nuttx &
     % ./nuttx
 
-NOTE: The unix domain socket path (/tmp/vpnkit-nuttx) is
-currently hardcoded in the code.
+NuttX's CONFIG_SIM_NETDEV_VPNKIT_PATH should match vpnkit's
+--ethernet option.