Add function to write DNS entries

Added function to write DNS entries to specified file
Also, added test case

Change-Id: I64250c7ee3cf7db3e7f8b5cf1669c7b4a7738637
Signed-off-by: Vishwanatha Subbanna <vishwa@linux.vnet.ibm.com>
diff --git a/Makefile.am b/Makefile.am
index eac5692..e2f2fcf 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -22,7 +22,8 @@
 		dhcp_configuration.hpp \
 		vlan_interface.hpp \
 		rtnetlink_server.hpp \
-		timer.hpp
+		timer.hpp \
+		dns_updater.hpp
 
 phosphor_network_manager_SOURCES = \
 		ethernet_interface.cpp \
@@ -39,7 +40,8 @@
 		dhcp_configuration.cpp \
         vlan_interface.cpp \
 		rtnetlink_server.cpp \
-		timer.cpp
+		timer.cpp \
+		dns_updater.cpp
 
 CLEANFILES = \
 		xyz/openbmc_project/Network/VLAN/Create/server.cpp \