adding BUILDER option "script" to support upstream projects
with more than one Dockerfiles,like
https://github.com/intel/intel-device-plugins-for-kubernetes.
it also has ability to support more complex cases.

Options supported by BUILD=script include:
LABEL: the image name
SOURCE_REPO: main project source git repo
SOURCE_REF: git branch or tag for main project source repo
COMMAND: command to run script
SCRIPT: script path
ARGS: args of script (op)

adding OS option "distroless" to support upstream projects
whoes base image is distroless, such as "gcr.io/distroless/base"

Change-Id: Idc9ca03bf6113e554fd23ac9b6bce570866bafbc
Story: 2005937
Task: 35665
Signed-off-by: SidneyAn <ran1.an@intel.com>