phase1: use buildinfo instead of prepare

phase1: use buildinfo instead of prepare

Prepare target is used to compile tools and toolchain, however buildbot
only uses it to create the buildinfo files. This[0] PR splits prepare
and buildinfo allowing the buildbots to run the buildinfo creation only.

This should also fix the current issue with missing json info files:
prepare runs `target/compile` which removes all existing json info files
via `Device/Build`. Only `target/install` creates them via
`Device/Build/image`. However currently the buildbots run
`target/install` before (the second) `target/compile`.

[0]: https://github.com/openwrt/openwrt/pull/2492

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