odhcp6c: add skpriority option
Allowing the (kernel) packet priority to be set through UCI.
This makes it straightforward to set some VLAN priority for DHCP
requests through a simple egress qos map. (Avoiding the need for
firewall matching and marking through iptables, which prevents using
flow offloading).
(Such priority tag is a hard requirement for some ISPs, such as Orange
in France).
Depends on: https://github.com/openwrt/odhcp6c/pull/74
Signed-off-by: Pacien TRAN-GIRARD <pacien.trangirard@pacien.net>