build: don't include kernel/libc in package index
The same that is done in `ipkg-make-index.sh` should happen with APK.
If the pseudo packages, only added to add dependency constraints, are
added to the index, APK happily "upgrades" them and installs updated
kmods, too. However, the Kernel itself is never installed via a regular
package.
Fixes: https://github.com/openwrt/openwrt/issues/16808
Signed-off-by: Paul Spooren <[email protected]>
Paul Spooren