odhcp6c: add route-workaround for broken IPv6-ISPs

Some ISP seem to only do stateful DHCPv6 and not sending RAs.
This is technically broken because plain DHCPv6 doesn't carry routes.

We work around here by faking a default route to the DHCPv6 server
if we do not receive a useful RA from the ISP.

This workaround can be turned off with: option fakeroutes 0

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42803 3c298f89-4303-0410-b956-a3cf2f4a3e73