build: add buildinfo as single Makefile target

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>

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