Complete support for local signing keys for APK.
A local key will be always generated, mkndx is always called with
--allow-untrusted as it needs to replace the sign key with the new local
one.
With CONFIG_SIGNATURE_CHECK the local index is signed with the local
key. Local public key is added with the ADD_LOCAL_KEY option.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>