AP_Scripting: use generic access userdata for serial ports

Adding another layer instead of just exposing UARTDriver bindings allows
substitution of the different functions for device simulation later.

Also take the opportunity to rework the docs a little.

libraries/AP_Scripting/lua_bindings.cpp

@@ -655,10 +655,10 @@ int AP_HAL__I2CDevice_read_registers(lua_State *L) {
     return success;
 }
 
-int AP_HAL__UARTDriver_readstring(lua_State *L) {
+int AP_Scripting_SerialAccess_readstring(lua_State *L) {
     binding_argcheck(L, 2);
 
-    AP_HAL::UARTDriver * ud = *check_AP_HAL__UARTDriver(L, 1);
+    AP_Scripting_SerialAccess * p = check_AP_Scripting_SerialAccess(L, 1);
 
     const uint16_t count = get_uint16_t(L, 2);
     uint8_t *data = (uint8_t*)malloc(count);
@@ -666,7 +666,7 @@ int AP_HAL__UARTDriver_readstring(lua_State *L) {
         return 0;
     }
 
-    const auto ret = ud->read(data, count);
+    const auto ret = p->read(data, count);
     if (ret < 0) {
         free(data);
         return 0;
@@ -743,6 +743,30 @@ int lua_get_CAN_device2(lua_State *L) {
 }
 #endif // AP_SCRIPTING_CAN_SENSOR_ENABLED
 
+#if HAL_GCS_ENABLED
+int lua_serial_find_serial(lua_State *L) {
+    // Allow : and . access
+    const int arg_offset = (luaL_testudata(L, 1, "serial") != NULL) ? 1 : 0;
+
+    binding_argcheck(L, 1 + arg_offset);
+
+    uint8_t instance = get_uint8_t(L, 1 + arg_offset);
+
+    AP_SerialManager *mgr = &AP::serialmanager();
+    AP_HAL::UARTDriver *driver_stream = mgr->find_serial(
+        AP_SerialManager::SerialProtocol_Scripting, instance);
+
+    if (driver_stream == nullptr) { // not found
+        return 0;
+    }
+
+    new_AP_Scripting_SerialAccess(L);
+    check_AP_Scripting_SerialAccess(L, -1)->stream = driver_stream;
+
+    return 1;
+}
+#endif // HAL_GCS_ENABLED
+
 /*
   directory listing, return table of files in a directory
  */