strip path when untaring kernel for headers

This is needed when custom/snapshot kernel headers are selected.
Buildroot expects to find dl/linux-2.6.tar.bz2 and also expects
it to extract to output/toolchain/linux-2.6, but if you tar
custom kernel sources directory can be anything, etc. linux-git.
In that case buildroot would extract it to output/toolchain/linux-git.
So fix this behaviour, by using tar strip-directory/component.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>