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: NJan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

linux-headers/README

+Automatically imported Linux kernel headers.
+Only use scripts/update-linux-headers.sh to update!

scripts/update-linux-headers.sh

+#!/bin/sh -e
+#
+# Update Linux kernel headers QEMU requires from a specified kernel tree.
+#
+# Copyright (C) 2011 Siemens AG
+#
+# Authors:
+#  Jan Kiszka        <jan.kiszka@siemens.com>
+#
+# This work is licensed under the terms of the GNU GPL version 2.
+# See the COPYING file in the top-level directory.
+
+tmpdir=`mktemp -d`
+linux="$1"
+output="$2"
+
+if [ -z "$linux" ] || ! [ -d "$linux" ]; then
+    cat << EOF
+usage: update-kernel-headers.sh LINUX_PATH [OUTPUT_PATH]
+
+LINUX_PATH      Linux kernel directory to obtain the headers from
+OUTPUT_PATH     output directory, usually the qemu source tree (default: $PWD)
+EOF
+    exit 1
+fi
+
+if [ -z "$output" ]; then
+    output="$PWD"
+fi
+
+for arch in x86 powerpc s390; do
+    make -C "$linux" INSTALL_HDR_PATH="$tmpdir" SRCARCH=$arch headers_install
+
+    rm -rf "$output/linux-headers/asm-$arch"
+    mkdir -p "$output/linux-headers/asm-$arch"
+    for header in kvm.h kvm_para.h; do
+        cp "$tmpdir/include/asm/$header" "$output/linux-headers/asm-$arch"
+    done
+    if [ $arch = x86 ]; then
+        cp "$tmpdir/include/asm/hyperv.h" "$output/linux-headers/asm-x86"
+    fi
+done
+
+rm -rf "$output/linux-headers/linux"
+mkdir -p "$output/linux-headers/linux"
+for header in kvm.h kvm_para.h vhost.h virtio_config.h virtio_ring.h; do
+    cp "$tmpdir/include/linux/$header" "$output/linux-headers/linux"
+done
+if [ -L "$linux/source" ]; then
+    cp "$linux/source/COPYING" "$output/linux-headers"
+else
+    cp "$linux/COPYING" "$output/linux-headers"
+fi
+
+rm -rf "$tmpdir"