Homogenize reachover build file structure with that of kyua-cli:

- Move the majority of the common build definitions to the top-level
Makefile.inc and ensure this gets included everywhere.
- Move

Homogenize reachover build file structure with that of kyua-cli:

- Move the majority of the common build definitions to the top-level
Makefile.inc and ensure this gets included everywhere.
- Move the bconfig.h file to the top-level directory.
- Add a statically-generated defs.h file instead of creating one
during the build. Easier to understand and less chances for things
to go wrong.
- Make sure all files using ATF_VERSION have the right dependency to
trigger a rebuild when the value changes.
- Clean up stale -I flags.

This is all mostly for simplicity reasons and to reduce the cognitive
load required to understand the build of the atf and kyua-* packages.

I have tested this with both MKKYUA=no/yes and non-clean/clean builds
so hopefully I got the details right. But if not, let me know please.

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