package/udev: create base device nodes after mounting a new /dev

Currently, udev won't start for me, as we've just mounted a new /dev,
which is completely empty, and udevd requires a /dev/null.

This change manually creates the three base device nodes (zero, null
and console) so that udev can start.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>