cpupower: make systemd unit installation optional

cpupower: make systemd unit installation optional

cpupower currently installs a cpupower.service unit file into unitdir
unconditionally, regardless of whether systemd is used by the host.

Improve the installation procedure by making this systemd step optional:
a 'SYSTEMD' build parameter that defaults to 'true' and can be set to
'false' to disable the installation of systemd's unit file.

Since 'SYSTEMD' defaults to true, the current behavior is kept as the
default.

Link: https://lore.kernel.org/r/20260113132753.1730020-2-joaomarcos.costa@bootlin.com
Signed-off-by: João Marcos Costa <joaomarcos.costa@bootlin.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>