gzip: fix installation path

Make gzip install binaries to / rather than /usr to fix bug #7766, it's
the FHS mandated target.

This also avoids duplicating binaries with busybox when both are
installed.

Also make gzip install after busybox if both are enabled to make the
proper gzip package override any busybox version since it's usually more
lightweight in functionality and slower.

And add a hash file while at it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>