AP_Scripting: example showing how to use readstring method

a fairly common requirement for scripts

libraries/AP_Scripting/examples/readstring_test.lua

@@ -0,0 +1,50 @@
+--[[
+   example demonstrating how to read from a serial port into a lua string
+--]]
+
+local baud_rate = 57600
+
+local port = assert(serial:find_serial(0), "Could not find Scripting Serial Port")
+
+port:begin(baud_rate)
+port:set_flow_control(0)
+
+--[[
+  get a string by concatenating bytes from the serial port,
+  suitable for ArduPilot 4.4.x
+--]]
+local function get_string_44(n)
+   local ret = ""
+   for _ = 1, n do
+      local b = port:read()
+      ret = ret .. string.char(b)
+   end
+   return ret
+end
+
+--[[
+  get a string directly uisng readstring
+  suitable for ArduPilot 4.5.x and later
+--]]
+local function get_string_45(n)
+   return port:readstring(n)
+end
+
+function update()
+   -- test using 4.5 method
+   local n = port:available():toint()
+   if n > 0 then
+      local str = get_string_45(n)
+      gcs:send_text(0, string.format("Received: '%s'", str))
+   end
+
+   -- test using 4.4 method (just so we don't have an unused function in lua check)
+   n = port:available():toint()
+   if n > 0 then
+      local str = get_string_44(n)
+      gcs:send_text(0, string.format("Received: '%s'", str))
+    end
+   return update, 100
+end
+
+return update, 100