metal 1.0: kernel base file includes headers visible to user code

[pgkeller]

This is a build problem and an API problem, may need to split in two.

Kernel builds build a wrapper, eg, brisck.cc to launch the user supplied kernel.
Internally, I believe the user supplied kernel gets catenated w/ the wrapper.
This means header files included in the wrapper get included in the client kernel by default.
For example, removing dataflow_api.h in idle_erisck.cc results in user kernel build failures.
The wrapper headers should be cleaned up/reduced, but this dependency causes grief, so better to break the dependency and clean up later. This means fixing the build first.

fyi: @eyonland