Layering hostname/hosts (#5)

* Layering hostname/hosts

* Fix build

* Use origin files on new overlay

buildroot-external/rootfs-overlay/usr/lib/systemd/system/etc-hostname.mount

@@ -0,0 +1,14 @@
+[Unit]
+Description=Hostname persistent configuration
+Requires=mnt-overlay.mount
+After=mnt-overlay.mount systemd-tmpfiles-setup.service
+Before=network.target
+
+[Mount]
+What=/mnt/overlay/etc/hostname
+Where=/etc/hostname
+Type=none
+Options=bind
+
+[Install]
+WantedBy=hassio-bind.target