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

tools/power/x86/x86_energy_perf_policy/Makefile

+DESTDIR ?=
+
 x86_energy_perf_policy : x86_energy_perf_policy.c
 
 clean :
 	rm -f x86_energy_perf_policy
 
 install :
-	install x86_energy_perf_policy /usr/bin/
-	install x86_energy_perf_policy.8 /usr/share/man/man8/
+	install x86_energy_perf_policy ${DESTDIR}/usr/bin/
+	install x86_energy_perf_policy.8 ${DESTDIR}/usr/share/man/man8/