piegen: separates generated output from debug

Introduce a new -o argument to piegen to specify generate file name.
Send the debug stream to stdout and force it to /dev/null in the makefile
if V=1 is not specify.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>