Allow to build P4 tools without debug flags to improve BMv2 performance
Allow to build P4 tools without debug flags to improve BMv2 performance

If the env variable DEBUG_FLAGS is set to false, build tools without
debug features to improve throughput of BMv2 and reduce CPU/memory
footprint.

Debug features include BMv2 logging, debugger, nanomsg, etc.

With DEBUG_FLAGS=true, when running 20 BMv2 instances, it requires 4 CPU
cores 100%. With DEBUG_FLAGS=false, when running 50 BMv2 instances,
overall CPU usage is only 1%.

Change-Id: Ifdd8ea976940b440797beb0e2300a749a55907dd

gkatsikas/onos,oplinkoms/onos,gkatsikas/onos,oplinkoms/onos,oplinkoms/onos,gkatsikas/onos,oplinkoms/onos,opennetworkinglab/onos,opennetworkinglab/onos,kuujo/onos,kuujo/onos,kuujo/onos,opennetworkinglab/onos,gkatsikas/onos,gkatsikas/onos,gkatsikas/onos,opennetworkinglab/onos,oplinkoms/onos,opennetworkinglab/onos,kuujo/onos,kuujo/onos,kuujo/onos,opennetworkinglab/onos,oplinkoms/onos,oplinkoms/onos,kuujo/onos