rtu: add mock modbus test device

Add the mocked modbus test device/server. The purpose of this mocked
device is to help facilitate testing on Qemu VM. More details are
captured in Readme.md file.

Change-Id: I3b09f4057ea5cc03db101b81f435ae772d51d601
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/meson.build b/meson.build
index 643b7ec..060e6f0 100644
--- a/meson.build
+++ b/meson.build
@@ -25,6 +25,10 @@
     subdir('rtu')
 endif
 
+if get_option('mocked-test-device').allowed()
+    subdir('mocked_test_device')
+endif
+
 if get_option('tests').allowed()
     subdir('tests')
 endif