config/powerpc: Always call update_network_config

Currently, we have a bug when setting the default network configuration
(ie, no devices have any specific config), as we won't call
update_network_config to clear out the value of the petitboot,network
parameter.

This change always invokes update_network_config, regardless of the
number of interfaces configured. This ensures we clear out the
petitboot,network param, which is then synced to nvram if the param has
changed.

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