kirkwood-kernel: disable NAND and virtual server

Change kernel config to deactivate NAND support and virtual server
support of netfilter. The latter was done because it is unneeded
and gave obsolete option warnings.

Signed-off-by: Angelika Tobisch <tobisch@dresearch.de>
Acked-by: Eik Binschek <binschek@dresearch-fe.de>