config: introduce separate CONFIG_SIGNATURE_CHECK option

config: introduce separate CONFIG_SIGNATURE_CHECK option

Introduce a new option CONFIG_SIGNATURE_CHECK which defaults to the value
of CONFIG_SIGNED_PACKAGES and thus is enabled by default.

This option is needed to support building target opkg with enabled
signature verification while having the signed package lists disabled.

Our buildbots currently disable package signing globally in the
buildroot and SDK to avoid the need to ship private signing keys to
the build workers and to prevent the triggering of random key generation
on the worker nodes since package signing happens off-line on the master
nodes.

As unintended side-effect, updated opkg packages will get built with
disabled signature verification, hence the need for a new override option.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>