kheaders: rebuild kheaders_data.tar.xz when KBUILD_BUILD_TIMESTAMP is changed

This problem is similar to commit 7f8256ae0e ("initramfs: Encode
dependency on KBUILD_BUILD_TIMESTAMP"): kernel/gen_kheaders.sh has an
internal dependency on KBUILD_BUILD_TIMESTAMP that is not exposed to
make, so changing KBUILD_BUILD_TIMESTAMP will not trigger a rebuild
of the archive.

Move $(KBUILD_BUILD_TIMESTAMP) to the Makefile so that is is recorded
in the *.cmd file.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

kernel/Makefile

@@ -162,7 +162,7 @@ $(obj)/config_data: $(KCONFIG_CONFIG) FORCE
 $(obj)/kheaders.o: $(obj)/kheaders_data.tar.xz
 
 quiet_cmd_kheaders_data = GEN     $@
-      cmd_kheaders_data = "$<" "$@" "$(obj)/kheaders-srclist" "$(obj)/kheaders-objlist"
+      cmd_kheaders_data = "$<" "$@" "$(obj)/kheaders-srclist" "$(obj)/kheaders-objlist" "$(KBUILD_BUILD_TIMESTAMP)"
       cmd_kheaders_data_dep = cat $(depfile) >> $(dot-target).cmd; rm -f $(depfile)
 
 define rule_kheaders_data

kernel/gen_kheaders.sh

@@ -7,6 +7,7 @@ set -e
 tarfile=$1
 srclist=$2
 objlist=$3
+timestamp=$4
 
 dir=$(dirname "${tarfile}")
 tmpdir=${dir}/.tmp_dir
@@ -42,7 +43,7 @@ xargs -0 -P8 -n1 \
 rm -f "${tmpdir}.contents.txt"
 
 # Create archive and try to normalize metadata for reproducibility.
-tar "${KBUILD_BUILD_TIMESTAMP:+--mtime=$KBUILD_BUILD_TIMESTAMP}" \
+tar "${timestamp:+--mtime=$timestamp}" \
     --owner=0 --group=0 --sort=name --numeric-owner --mode=u=rw,go=r,a+X \
     -I $XZ -cf $tarfile -C "${tmpdir}/" . > /dev/null