[AETHER-544][AETHER-523] Improve versioning of `tost` docker image and simplify build workflow

VERSION file is used for the release/tag process of `tost`.

Makefile.vars file defines the stable versions of each component
to be used during the build process.

Dockerfile.tost defines a docker label for each component
and uses the *_VERSION vars to set the docker labels.

Moves build logic inside app-build.sh. Build process is stopped if there are pending
changes in the repositories

Adds check-scripts target for shellcheck verify

Add fetch targets to update the repos

Deprecates the use of git-review for git fetch ref/changes/

Updates README to explain the new build process

Change-Id: I9a3f4b1b67064cd55c755330deaf6efef19ced22
diff --git a/Dockerfile.tost b/Dockerfile.tost
index c4ecc44..5345b6e 100644
--- a/Dockerfile.tost
+++ b/Dockerfile.tost
@@ -63,7 +63,13 @@
 ARG org_label_schema_vcs_url=unknown
 ARG org_label_schema_vcs_ref=unknown
 ARG org_label_schema_build_date=unknown
-ARG org_onosproject_vcs_commit_date=unknown
+ARG org_onosproject_onos_version=unknown
+ARG org_onosproject_trellis_control_version=unknown
+ARG org_onosproject_trellis_t3_version=unknown
+ARG org_opencord_fabric_tofino_version=unknown
+ARG org_omecproject_up4_version=unknown
+ARG org_opencord_kafka_onos_version=unknown
+ARG org_stratumproject_fabric_tna_version=unknown
 
 LABEL org.label-schema.schema-version=1.0 \
       org.label-schema.name=tost \
@@ -71,4 +77,10 @@
       org.label-schema.vcs-url=$org_label_schema_vcs_url \
       org.label-schema.vcs-ref=$org_label_schema_vcs_ref \
       org.label-schema.build-date=$org_label_schema_build_date \
-      org.onosproject.vcs-commit-date=$org_onosproject_vcs_commit_date
+      org.onosproject.onos.version=$org_onosproject_onos_version \
+      org.onosproject.trellis-control.version=$org_onosproject_trellis_control_version \
+      org.onosproject.trellis-t3.version=$org_onosproject_trellis_t3_version \
+      org.opencord.fabric-tofino.version=$org_opencord_fabric_tofino_version \
+      org.omecproject.up4.version=$org_omecproject_up4_version \
+      org.opencord.kafka-onos.version=$org_opencord_kafka_onos_version \
+      org.stratumproject.fabric-tna.version=$org_stratumproject_fabric_tna_version