make: Be able to force turning off piegen

For testing purpose we need to disable using of
piegen utility. So lets add PIEGEN make option
thus one can "PIEGEN=no make" to build criu
without piegen at all.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>