cpupower: speed up generating git version string

The variable VERSION is expanded for every use of CFLAGS. This causes
"git describe" to get called multiple times on the kernel tree, which
can be quite slow.

The git revision does not change during build, so we can use simple
variable expansion to set VERSION.

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Acked-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>