Add kernel header update script

Add kernel header update script

This helper pulls the required kernel headers for KVM and vhost into a
specified directory. The update is triggered via

    scripts/update-linux-headers.sh LINUX_PATH

and will place the output under linux-headers/linux and linux-headers/asm-*.
It also imports the COPYING to care for headers without an explicit license.

CC: Alexander Graf <agraf@suse.de>
CC: Christoph Hellwig <hch@lst.de>
CC: Peter Maydell <peter.maydell@linaro.org>
CC: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>