build: opkg-key variable key folder

build: opkg-key variable key folder

The key folder is used by `opkg` and `usign` to store and retrieve
trusted public keys. Using `opkg-key` outside a running device is
unfeasible as the key folder is hard coded to `/etc/opkg/keys`.

This commit adds a variable OPKG_KEYS which defaults to `/etc/opkg/keys`
if unset, however allows set arbitrary key folder locations.

Arbitrary key folder locations are useful to add signature verification
to the ImageBuilders.

Signed-off-by: Paul Spooren <mail@aparcar.org>