Adds a new variable DISABLED_SERVICES to ImageBuilder Makefile, which
defines a list of services (installed as /etc/init.d/*) to be disabled
during the build of a custom image (normally all are enabled).
It comes handy when a particular service should not be run under normal
circumstances, but should be ready in the image for situations when it
might be needed.
Signed-off-by: Richard Musil <risa2000x@gmail.com>