base-files: prevent issues w/ overlay on powerloss after sysupgrade

Due to filesystem write caching the old configuration data could stay
out of flash for a long time during a first boot after the sysupgrade.
Power loss during this period could damage the overlay data and even
make device inaccessable via the network.

Fix this by syncing data to a flash as soon as the previous
configuration will be unpacked after the sysupgrade. Also sync the FS
state after the sysupgrade.tgz archive removing to prevent duplicative
extraction of a previous configuration.

Tested with AMD Geode based board.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>

package/base-files/files/etc/init.d/done

@@ -4,7 +4,7 @@
 START=95
 boot() {
 	mount_root done
-	rm -f /sysupgrade.tgz
+	rm -f /sysupgrade.tgz && sync
 
 	# process user commands
 	[ -f /etc/rc.local ] && {

package/base-files/files/lib/preinit/80_mount_root

@@ -9,6 +9,8 @@ do_mount_root() {
 		echo "- config restore -"
 		cd /
 		tar xzf /sysupgrade.tgz
+		# Prevent configuration corruption on a power loss
+		sync
 	}
 }