Allow overriding sysctl parameters via /etc/sysctl.d files (#3883)

Relocate current content of /etc/sysctl.d to /usr/lib and make the /etc folder
writable via a bind mount.

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

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