fs/common.mk: do not store original names and timestamps when creating gzipped rootfs

fs/common.mk: do not store original names and timestamps when creating gzipped rootfs

Using the GZIP environment variable to pass gzip options is
deprecated, and therefore we are going to remove the "GZIP = -n"
definition from the main Buildroot Makefile. In preparation for this,
we explicitly add the -n argument to the gzip call in fs/common.mk to
ensure reproducibility.

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>