Using a relative path has the advantage that when the kernel source
tree is moved the relevant .o files will not be rebuild just because
the path to the kernel src has changed.
This also got rid of a user of TOPDIR - which has been deprecated for a long time now.
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!