Add a manual sleep and rootdelay

It seems udev trigger/settle doesn't do what we expect,
so we'll add functionality for the rootdelay param back
in here.

This should help fix FS#12896

Original-idea: Gerhard Brauer <gerbra@archlinux.de>
Signed-off-by: Aaron Griffin <aaronmgriffin@gmail.com>

archiso/hooks/archiso

@@ -57,6 +57,10 @@ run_hook ()
     msg ":: Waiting for usb devices to settle..."
     /sbin/udevadm trigger --subsystem-match=usb
     /sbin/udevadm settle
+    if [ "${rootdelay}" != "0" ]; then
+        /bin/sleep "${rootdelay}"
+        export rootdelay=0
+    fi
 
     msg ":: Scanning for boot device..."