In the next patches, the software/virtual drivers (slcan, v(x)can)
will depend on drivers/net/can/dev/skb.o.

This patch changes the scope of the can-dev module to include the
above mentioned drivers.

To do so, we reuse the menu "CAN Device Drivers" and turn it into a
configmenu using the config symbol CAN_DEV (which we released in
previous patch). Also, add a description to this new CAN_DEV
menuconfig.

The symbol CAN_DEV now only triggers the build of skb.o. For this
reasons, all the macros from linux/module.h are deported from
drivers/net/can/dev/dev.c to drivers/net/can/dev/skb.c.

Finally, drivers/net/can/dev/Makefile is adjusted accordingly.

Link: https://lore.kernel.org/all/20220610143009.323579-3-mailhol.vincent@wanadoo.frSuggested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Acked-by: Max Staudt <max@enpas.org>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>