cpupower tool: allow to build in a separate directory

This patch allows cpupower tool to generate its output files in a
seperate directory. This is now possible by passing the 'O=<path>' to
the command line.

This can be usefull for a normal user if the kernel source code is
located in a read only location.

This is patch stole some bits of the perf makefile.

[linux@dominikbrodowski.net: fix commit message]
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>