odhcp6c: Make SLAAC-only work without reqprefix=no

Previously if the upstream router did not provide any DHCPv6-service
the wan6 interface would not come up even if RAs were received. The new
behavior allows the wan6 interface to comeup with RA-only configuration
after a timeout of 10s.

git-svn-id: svn://svn.openwrt.org/openwrt@36555 3c298f89-4303-0410-b956-a3cf2f4a3e73