tools: Allow tools to be installed in a user specified location

When building x86_energy_perf_policy or turbostat within the confines of
a packaging system such as RPM, we need to be able to have it install to
the buildroot and not the root filesystem of the build machine.  This
adds a DESTDIR variable that when set will act as a prefix for the
install location of these tools.

Signed-off-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>