Makefile: add '-n' to gzip invocations to improve reproducibility

Makefile: add '-n' to gzip invocations to improve reproducibility

Default invocation to gzip include timestamp in output file. This feature is
incompatible with BR2_REPRODUCIBLE. It is possible to disable it with '-n'.

The environment variable GZIP can hold a set of default options for gzip. So
instead to find all gzip invocation in build process, we just export 'GZIP=-n'.

Notice bzip2, lzma and xz are not impacted by this problem. On the other hand, lzop
does include timestamp and does not provide any way to disable it.

This work was sponsored by `BA Robotic Systems'.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>