net: support NETCONSOLE option via Kconfig

net: support NETCONSOLE option via Kconfig

This patch introduces CONFIG_NETCONSOLE as an option to the
Kconfig system.

Joe Hershberger pointed out that it may not be entirely free of
problems, as many boards predating the driver model define this
symbol directly via include files. In case they're not properly
migrated, their NetConsole might 'vanish' if they start to use
CONFIG_NET or CONFIG_NETDEVICES.

Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>