Makefile: add bldr-kill target to kill all remaining bldr processes
Makefile: add bldr-kill target to kill all remaining bldr processes

After running `make bldr-run` or `make bldr-run-no-build`, all bldr
processes as well as other hab processes keep running. They are not
killed even after stopping the make process itself, which in the next
run prevents bldr from running at all. So we need to have another
target in Makefile, which allows killing every running process in the
background. Also we need to unmount node_modules directories that are
mounted by `builder_web.sh`.

Signed-off-by: Dongsu Park <696fb7639d678a1e692fa501217d7509545d7331@kinvolk.io>

rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat