hooks/set-stdout: move hook to after automatic stdout settings

If a console is defined in NVRAM, we want it to take precedence over
anything that 30-add-offb calculates automatically. This change shifts
the 20-set-stdout script to later in the hook processing, so that it
will override any automatic settings.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>