The prepare target was added some 11 years ago to build tools and
toolchain and was recently extended to  create buildinfo files for
reproducibility, meaning {feeds,version,config}.buildinfo.

As the buildbot workflow is more complex than the single prepare (kmod
feed insertion), prepare is only used to create those buildinfo files.
Running prepare however runs `target/compile` as well, taking time even
everything is already compiled.

Splitting this allows the buildbot to run only the `buildinfo` target
while others can still use the convenience feature `prepare`.

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