linux: enable all options needed by xtables-addons

Both of CONFIG_NF_CONNTRACK and CONFIG_NF_CONNTRACK_MARK are needed by
xtables-addons.

Although the current code does enable them in the linux' .config file,
the former is protected behind CONFIG_NETFILTER_ADVANCED, which may be
missing from a user-supplied (def)config file, and is missing from some
of the bundled defconfigs as well.

For example, the following defconfig fails to build:

    BR2_TOOLCHAIN_EXTERNAL=y
    BR2_LINUX_KERNEL=y
    BR2_LINUX_KERNEL_DEFCONFIG="i386"
    BR2_PACKAGE_XTABLES_ADDONS=y

So, also force-enable CONFIG_NETFILTER_ADVANCED.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>