fs/tar: explicitly set extended header values to ensure binary reproducibility

Since we use --xattrs-include='*' to include all extended attributes,
tar creates a PAX formatted archive. The archive metadata captures atime
and ctime of files. To fix this, GNU recommends that we pass this added
argument to tar to create binary reproducible packages. Setting of mtime
is handled in fs/common.mk using touch on all files.

Diffoscope output pre-change: https://gitlab.com/snippets/1871111
Diffoscope output after change is blank i.e. binary reproducibile rootfs
is created.

GNU Recommendation: https://www.gnu.org/software/tar/manual/tar.html#SEC147

Signed-off-by: Atharva Lele <itsatharva@gmail.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

fs/tar/tar.mk

@@ -8,6 +8,9 @@ TAR_OPTS := $(call qstrip,$(BR2_TARGET_ROOTFS_TAR_OPTIONS))
 
 ROOTFS_TAR_DEPENDENCIES = $(BR2_TAR_HOST_DEPENDENCY)
 
+# do not store atime/ctime in PaxHeaders to ensure reproducbility
+TAR_OPTS += --pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0,ctime:=0
+
 define ROOTFS_TAR_CMD
 	(cd $(TARGET_DIR); find -print0 | LC_ALL=C sort -z | \
 		tar $(TAR_OPTS) -cf $@ --null --xattrs-include='*' --no-recursion -T - --numeric-owner)