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>

scripts/package/mkspec

@@ -39,7 +39,7 @@ if ! $PREBUILT; then
 echo "Source: kernel-$__KERNELRELEASE.tar.gz"
 fi
 
-echo "BuildRoot: /var/tmp/%{name}-%{PACKAGE_VERSION}-root"
+echo "BuildRoot: %{_tmppath}/%{name}-%{PACKAGE_VERSION}-root"
 echo "Provides: $PROVIDES"
 echo "%define __spec_install_post /usr/lib/rpm/brp-compress || :"
 echo "%define debug_package %{nil}"