* Updated Makefile infrastructure to build debug images.
As a sample, platform/broadcom/docker-orchagent-brcm.mk is updated to add a docker-orchagent-brcm-dbg.gz target.
Now "BLDENV=stretch make target/docker-orchagent-brcm-dbg.gz" will build the debug image.
This debug image can be used in any linux box to inspect core file. If your module's external dependency can be suitably mocked, you my even manually run it inside.
"docker run -it --entrypoint=/bin/bash e47a8fb8ed38"
You may map the core file path to this docker run.
* Dropped the regular binary using DBG_PACKAGES and a small name change to help readability.
* Tweaked the changes to retain the existing behavior w.r.t INSTALL_DEBUG_TOOLS=y.
When this change ('building debug docker image transparently') is extended to all dockers, this flag would become redundant. Yet, there can be some test based use cases that rely on this flag.
Until after all the dockers gets their debug images by default and we switch all use cases of this flag to use the newly built debug images, we need to maintain the existing behavior.
