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.options b/meson.options
index 5346c2b..eee0609 100644
--- a/meson.options
+++ b/meson.options
@@ -5,4 +5,11 @@
     description: 'Enable modbus RTU.',
 )
 
-option('tests', type: 'feature', description: 'Build tests')
+option(
+    'mocked-test-device',
+    type: 'feature',
+    value: 'enabled',
+    description: 'Build mocked modbus devuce.',
+)
+
+option('tests', type: 'feature', description: 'Build tests.')