Makefile: don't export GZIP environment variable

Makefile: don't export GZIP environment variable

We export GZIP = -n so that GZIP does not record original
name and timestamps. However..

GZIP environment variable is deprecated and soon will not be
supported in future GZIP versions. GZIP suggests the use of a
wrapper to pass options globally but it might be difficult to
implement in Buildroot. For now, we don't export the variable
and fix reproducibility issues per package as they show up in
Autobuilder.

Signed-off-by: Atharva Lele <itsatharva@gmail.com>
Acked-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>