gre: make encaplimit support configurable

gre: make encaplimit support configurable

Make inclusion of the destination option header containing the tunnel
encapsulation limit configurable for IPv6 GRE packets.
Setting the uci parameter encaplimit to ignore; allows to disable the
insertion of the destination option header in the IPv6 GRE packets.
Otherwise the tunnel encapsulation limit value can be set to a value
from 0 till 255 by setting the encaplimit uci parameter accordingly.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
(backported from 3d015e971f5e3f0df8e8ab149fda1270c5c72507)