download/git: create GNU format tar files

download/git: create GNU format tar files

On most distros, the tar format defaults to GNU. However, at build time
the default format may be changed to posix. Also, future versions of
tar will default to posix.

Since we want the tarballs created by the git download method to be
reproducible (so their hash can be checked), we should explicitly
specify the format. Since existing tarballs on sources.buildroot.org
use the GNU format, and also the existing hashes in the *.hash files
are based on GNU format tarballs, we use the GNU format.

In addition, the Posix format encodes atime and ctime as well as mtime,
but tar offers no option like --mtime to override them. In the GNU
format, atime and ctime are only encoded if the --incremental option is
given.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>