add rtu and common dirs

Add rtu (remote terminal unit) and common dirs and enable this in top
level meson.build file. Add meson option for modbus-rtu and
conditionally include rtu based on this option.

Tested: meson build passed.

Change-Id: I9b3b061cf31ebb5195854f89ce399bc765c64a85
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/meson.build b/meson.build
index d73b2e1..9bd3146 100644
--- a/meson.build
+++ b/meson.build
@@ -16,3 +16,9 @@
     phosphor_logging_dep,
     sdbusplus_dep,
 ]
+
+subdir('common')
+
+if get_option('modbus-rtu').allowed()
+    subdir('rtu')
+endif