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/rtu/meson.build b/rtu/meson.build
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/rtu/meson.build
@@ -0,0 +1 @@
+