Avoid running a helper program for atf-sh as a test.

Should fix failures observed in PR bin/48624.

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.

Update reachover build files for atf-0.20.

branches: 1.5.6; 1.5.12;
Adjust reachover makefiles for atf 0.11.

Set ATFFILE=yes to install the package-provided Atffiles instead of generating
our custom ones.

Update reachover makefiles for atf 0.10.

Adjust reachover makefiles for atf 0.9.

Add reachover Makefiles to build ATF 0.6. These replace the old Makefiles
that were spread all around the tree when ATF lived in dist/atf.

Rebase to HEAD as of a few days ago.

sync with head.

for a reference, the tree before this commit was tagged
as yamt-pagecache-tag8.

this commit was splitted into small chunks to avoid
a limitation of cvs. ("Protocol error: too many arguments")