wireguard: Add option to disable Endpoint Host Dependency

When using IPv6 addresses (or DNS which has AAAA record) as endpoint,
add_host_dependency fails for some reason which causes the wireguard
interface to disappear.

This patch backports an option from OpenWrt master, where
host_dependency can be disabled by adding 'option nohostroute 1'
to the interface.

This is an alternate approach based on the work of Fabian Bläse.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>