kernel-devsrc: fix reproducibility and buildpaths QA warning

We've been removing .vdso-offsets.h.cmd for quite some time, as it
is not required to regenerate scripts or to satisfy the prepare
target in the kernel source.

That being said, there is a secondary location that this file may
be found, and since we aren't detecting or removing it, we end up
with embedded build paths, which cause both reproducibility and
buildpaths QA warnings.

We can test for both files, and force remove them if present, and
we'll fix this new location of the file and not risk breaking
any old kernels.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>