imagebuilder: fix package inclusion and APK index

imagebuilder: fix package inclusion and APK index

We need to exclude packages from the APK index which must not be
upgraded. To do so, the packages `libc`, `kernel` and `base-files` are
excluded to APK never suggestes them as upgradable.

The previous logic would however match packages like `libcomerr0`,
causing build failures. Make the copying and excluding logic more
precise by adding a single dash.

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