scripts: use %_tmppath in "make rpm-pkg"

The mkspec script hardcodes "/var/tmp" into the generated rpm spec file's
BuildRoot. The user, however, may have a custom setting for %_tmppath,
which should be used in BuildRoot.  This patch changes mkspec's
BuildRoot output to appropriately use %_tmppath.

Signed-off-by: John Saalwaechter <saalwaechter@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>