[first boot] sync file system after moving/copying files (#3550)

Signed-off-by: Ying Xie <ying.xie@microsoft.com>

files/image_config/platform/rc.local

@@ -242,6 +242,8 @@ if [ -f $FIRST_BOOT_FILE ]; then
         dpkg -i /host/image-$SONIC_VERSION/platform/$platform/*.deb
     fi
 
+    sync
+
     # If the unit booted into SONiC from another NOS's grub,
     # we now install a grub for SONiC.
     if [ -n "$onie_platform" ] && [ -n "$migration" ]; then

files/image_config/updategraph/updategraph

@@ -33,6 +33,8 @@ function copy_config_files_and_directories()
             logger "Missing SONiC configuration ${file_dir} ..."
         fi
     done
+
+    sync
 }
 
 function check_system_warm_boot()