android: android-base.config: enable IP6_NF_MATCH_RPFILTER

USB tethering has a hard dependency on ip6t_rpfilter module now.
So enable IP6_NF_MATCH_RPFILTER config to make it work again,
otherwise we run into following failures when we try to enable
USB tethering on Hikey:

W IptablesRestoreController: iptables-restore process 1893 terminated status=256
E IptablesRestoreController: iptables error:
E IptablesRestoreController: ------- COMMAND -------
E IptablesRestoreController: *raw
E IptablesRestoreController: -A natctrl_raw_PREROUTING -i usb0 -m rpfilter --invert ! -s fe80::/64 -j DROP
E IptablesRestoreController: COMMIT
E IptablesRestoreController:
E IptablesRestoreController: ------- ERROR -------
E IptablesRestoreController: ip6tables-restore: line 319 failed
E IptablesRestoreController: ----------------------
E NatController: Error setting forward rules
E Tethering: [usb0] ERROR Exception enabling NAT: java.lang.IllegalStateException: command '55 nat enable
  usb0 wlan0 1 192.168.42.0/24' failed with '400 55 Nat operation failed (No such device)'

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>