Add missed option definition - root user mgmt.

Added missed option definition to enable / disable managing
root user under phosphor-user-manager. Default root user is managed
by phosphor-user-manager, but can be disbaled if needed.

Tested:
1. Verified that root user is listed as an object,
in default build
2. Verified that root user is not listed as an object with
EXTRA_OECONF += "--disable-root_user_mgmt"

Change-Id: Iaf677f36b7cc28b67977881235bd72915943b372
Signed-off-by: Richard Marian Thomaiyar <richard.marian.thomaiyar@linux.intel.com>
diff --git a/configure.ac b/configure.ac
index 5c6a6fe..43a4869 100644
--- a/configure.ac
+++ b/configure.ac
@@ -93,6 +93,9 @@
 AC_DEFINE_UNQUOTED([LDAP_CONF_PERSIST_PATH], ["$LDAP_CONF_PERSIST_PATH"], \
     [path of directory having persisted LDAP configuration enabled property.])
 
+AC_ARG_ENABLE([root_user_mgmt],
+    AS_HELP_STRING([--disable-root_user_mgmt], [Disables managing root user. [default=enable]])
+)
 AS_IF([test "x$enable_root_user_mgmt" != "xno"],
     AC_MSG_NOTICE([Enable managing root user])
     [cpp_flags="-DENABLE_ROOT_USER_MGMT"]