ramips: allow to set switchdev by board in ramips_set_preinit_iface

ramips: allow to set switchdev by board in ramips_set_preinit_iface

This adds the option to determine switchdev by board when setting
preinit iface for failsafe. The patch reorganizes the code to use
functions for setting correct switchdev based on SOC and board,
which is supposed to improve readability and maintainability.

In this patch, the ramips_switchdev_from_board function is added
without specifying an actual device using it. This is meant to
make the life of device supporters waiting for merge easier, as
there is less to rebase and keep track of.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>