Rc.filter_configure_sync can try to set default route to dynamic

[phil.davis]

Probably also happens in previous versions, but while I see it on the console:

Apr  6 13:42:55 php-fpm[8228]: /rc.filter_configure_sync: The command '/sbin/route change -inet default dynamic' returned exit code '68', the output was 'route: bad address: dynamic'

The WAN was set to DHCP and was plugged in to the ISP device but had not received an IP yet from DHCP. I am guessing that rc.filter_configure_sync can charge on through and simply use the word "dynamic" from the WAN_DHCP config as if it is the upstream gateway. There should be a check somewhere in there that jumps over that if WAN_DHCP has not got a real upstream gateway address yet.