Make usb_modeswitch include directory writable (#3800)

The /etc/usb_modeswitch.d is present and empty but it can't be written to allow
user modification. Bind-mount it like other /etc folders to make it possible to
adjust usb_modeswitch config.

Fixes #3785

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

@@ -0,0 +1,14 @@
+[Unit]
+Description=USB modeswitch persistent directory
+Requires=mnt-overlay.mount
+After=mnt-overlay.mount
+Before=hassos-config.service
+
+[Mount]
+What=/mnt/overlay/etc/usb_modeswitch.d
+Where=/etc/usb_modeswitch.d
+Type=None
+Options=bind
+
+[Install]
+WantedBy=hassos-bind.target