package: do not sign individual APK packages

package: do not sign individual APK packages

APK, unlike OPKG, can sign individual packages and not just indexes.
Since OpenWrt uses a distributed build infrastructure and only the build
master owns the private keys, signing of individual buildworkers doesn't
work. Right now, each buildworker creates a temporary build key to sign
packages, then transmits the package index to the buildmaster for a
signature.

As a result, all individual packages contain a nonsensical signature,
making them harder to reproduce. This commit removes the individual
package signing.

Since APK requires signatures per default, explicitly allow installation
of unsigned packages during the build process.

The config option here is for historical reasons misleading,
SIGNED_PACKAGES refers to the package index, not the individual
packages.

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