linux: adjust kernel config according to the Buildroot configuration

We only adjust the configuration or ARM EABI and IPv6. The (more
complicated) initramfs case is handled in a separate commit. The user
is expected to take care of all other configuration details (like
having Netfilter enabled to make iptables work, etc.)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>