ifupdown: added -1 option to dhclient for dhcpv6

When dhclient is used with ifupdown and in the /etc/network/interfaces
file is something like e.g. "iface eth0 inet6 dhcp" the system does not
boot but hangs at the after dhcp configuration of IPv4 at the stage
where IPv6 should be configured.
The reason seems to be, that ifupdown calls the dhclient in a blocking
way which leads to a permanent block of the boot when no DHCPv6 server
is available.
A similar bug in Redhat:
https://bugzilla.redhat.com/show_bug.cgi?id=585047



[YOCTO #14117]

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>