support/download: generate even more reproducible tarballs

When we generate the taballs off a local working copy of a VCS tree,
the umask is the one that we enforce in out top-level Makefile.

However, it is possible that a user manually tinkers in said working
copy (e.g. to check an upstream bug fix, or regression). If the user
umask is different from the one Buildroot enfirces, such tinkering
can impact the mode bits of the files, even if their content is not
modified.

When we eventually need to create a tarball from said working copy,
the VCS (e.g. git) will only be interested in checking whether the
content of the files have changed before chcking them out, and will
not look at, and restore/fix the mode bits.

As a consequence, we may create non-reproducible archives.

We fix that by enforcing the mode bits on the files before we create
the tarball: we disable the write and execute bits, and only set the
execute bit if the user execute bit is set.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Vincent Fazio <vfazio@xes-inc.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 768f9f80f62c1da6e298c680f0f4bfa887f38c78)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/download/helpers

@@ -53,6 +53,9 @@ mk_tar_gz() {
     tmp="$(mktemp --tmpdir="$(pwd)")"
     pushd "${in_dir}" >/dev/null
 
+    # Enforce group/others mode bits
+    chmod -R go-wx+X .
+
     # Establish list
     find . -not -type d -and -not \( -false "${find_opts[@]}" \) >"${tmp}.list"
     # Sort list for reproducibility