kernel: Activate CONFIG_NET_SWITCHDEV in generic config

kernel: Activate CONFIG_NET_SWITCHDEV in generic config

The CONFIG_NET_SWITCHDEV option is needed by CONFIG_DSA and some other
options. It is boolean, we have to compile it into the kernel it self.
Activate it for all targets in the generic configuration, it is already
activated for most of them. This allows to install DSA drivers as a
module.

On the ramips/mt7620 target the kernel would grown by 4.5kB.

For some small targets which do not support a DSA switch by default the
option is deactivated.

Link: https://github.com/openwrt/openwrt/pull/17668
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>