Move timestamp and version files into 'generated' subdir

There is a rather subtle build problem where the build time stamp is not
updated for out-of-tree builds if there exists an in-tree build which
has a valid timestamp file. So if you do an in-tree build, then an
out-of-tree build your timestamp will not change.

The correct timestamp_autogenerated.h lives in the object tree, but it
is not always found there. The source still lives in the source tree and
when compiling version.h, it includes timestamp_autogenerated.h. Since
the current directory is always searched first, this will come from the
source tree rather than the object tree if it exists there. This affects
dependency generation also, which means that common/cmd_version.o will not
even be rebuilt if you have ever done an in-tree build.

A similar problem exists with the version file.

This change moves both files into the 'generated' subdir, which is already
used for asm-offsets.h. Then timestamp.h and version.h are updated to
include the files from there.

There are other places where these generated files are included, but I
cannot see why these don't just use the timestamp.h and version.h headers.
So this change also tidies that up.

I have tested this with in- and out-of-tree builds, but not SPL. I have
looked at various other options for fixing this, including sed on the dep
files, -I- and -include flags to gcc, but I don't think they can be made
to work. Comments welcome.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>