kernel-devsrc: improve reproducibility for arm64

kernel-devsrc: improve reproducibility for arm64

.vdso-offsets.h.cmd contains command that was used to produce vdso-offsets.h.
It breaks reproducibility because it has an absolute path in it. There is no
any value to package such files so it can be dropped.

Signed-off-by: Denys Zagorui <dzagorui@cisco.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>