common: add entity manager interface

Define an EntityManager interface to fetch the serial port related
config or any device config from Entity Manager.

Change-Id: Ieeff4422903e3b4bcc3ce28cd493eecab5618f13
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/meson.build b/meson.build
index cbc46df..643b7ec 100644
--- a/meson.build
+++ b/meson.build
@@ -17,6 +17,8 @@
     sdbusplus_dep,
 ]
 
+common_include = include_directories('.')
+
 subdir('common')
 
 if get_option('modbus-rtu').allowed()