opkg: allow to configure the path to the signature verification script

opkg: allow to configure the path to the signature verification script

Currently, package index signatures are only checked when opkg runs on the
OpenWrt device.  The verification script is hard-coded to a path in
/usr/sbin/.

Making this path configurable is a first step to implement signature
verification in host builds of opkg (e.g. in the imagebuilder).

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>
Acked-by: Paul Spooren <mail@aparcar.org>