package/busybox: udhcpc.script: wait for ipv6 default route from RA for stateful...

package/busybox: udhcpc.script: wait for ipv6 default route from RA for stateful DHCPv6

udhcpc6 implements "stateful" DHCPv6 for explicitly requesting an address
and other configuration information. A major difference between DHCPv4
and DHCPv6 is that DHCPv6 does *not* advertise a default route; this is
determined by normal IPv6 autoconfiguration.

Add logic to wait up to IF_WAIT_DELAY seconds for the IPv6 route to be
configured; as above this doesn't come from DHCPv6 but rather the IPv6
Router Advertisement (RA) which happens independently from udhcpc6.  The
intent here is to try and ensure that the interface is route-able upon the
script's completion as it would be if called from udhcpc.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>