imagebuilder: use correct package directory when bundling kmods and libc

imagebuilder: use correct package directory when bundling kmods and libc

The libc and kernel package files moved since the introduction of shared
packages and the changed output directory layout. This causes the generated
ImageBuilder archive to lack the necessary "libc" and "kernel" meta packages,
leading to opkg install errors later on.

Use the FeedPackageDir macro to figure out the proper source directory to use.

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