Allow WNR3500Lv2 to upgrade from Web interface

Ignore error from erase IOCTL, image subsequently writes correctly.
Force prefix length of /64 in dnsmasq.conf

Will allow dnsmasq to start & perform RA on 1st all zeroes subnet
of </64 prefixes. Which may in turn reduce the headaches of those
blindly configuring /56 or /48 tunnels - at least some RA is done.
If it's wrong, then use dnsmasq.custom.  IPv6 is too big for us to
cope with every option :-)
manual 6RD gui - fix for typo tomato.c ipv6_6rd_prefix_length