linuxrc: call udevadm settle after modules_load

udev may still be processing rules and this can cause very bad
things. For instance, modules_load may have loaded an usb host
controller driver and we must wait for the udev rules to terminate.
However, this may lead to other race conditions, but we have
observed that adding scandelay=n where n >= 5 actually fixes the
issue of booting off USB under certain scenarios.