spec, Makefile: add /etc/nvme files to package files

spec, Makefile: add /etc/nvme files to package files

This way it can automatically be removed upon uninstallation
and we don't need %preun target. Also, this will also allow
to get these files on manual installation from sources.

Also, don't settle on the "file exists" check, use the '-s'
to check that it is not empty as well.

Lastly, in order to have hostnqn and hostid updated upon pkg
install, we have install-spec target that would omit hostnqn and
hostid files, and let the rpm %post do it (otherwise we
use the normal install target).

Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>