PR workflow: add mergify

This config should always follow our worklof and other way around. They should both be in sync.

What can mergify do? Check PR status (labels, reviewers) and act on it. First step would be to not merge anything, just use labels and help us to keep PRs up to date. Less manual work.

If this works, we can get CI to check specific labels to trigger CI (or use commands) and other things, this needs more work.

.mergify.yml

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+#
+# Copyright (c) 2020, Arm Limited and affiliates.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Mergify follow our workflow: https://os.mbed.com/docs/mbed-os/latest/contributing/workflow.html
+# Inital state (needs: review) is done by internal reviewer bot
+
+# Notes: conditions do not check if label is already there if we add or remove later, this is to limit Github API access to
+# minimum. If a label is already there and we add it again, nothing should happen (we do 1 less access to Github).
+# For instance for needs: work if CI failed, we just add a label as action, don't check if needs: works is already there
+
+pull_request_rules:
+  # From needs: review to needs: work - reviewers request
+  - name: "label needs: work when changes were requested"
+    conditions:
+      # Reviewers
+      - "#changes-requested-reviews-by>0"
+    actions:
+      label:
+        add: 
+          - "needs: work"
+        remove: 
+          - "needs: review"
+
+  # From needs: review to needs: work - CI failure
+  - name: "label needs: work when travis-ci failed"
+    conditions:
+      # Travis failing
+      - status-failure~=continuous-integration/travis-ci/pr
+    actions:
+      label:
+        add: 
+          - "needs: work"
+        remove: 
+          - "needs: review"
+
+  # From needs: review to needs: work - CI failure
+  - name: "label needs: work when Jenkins CI failed - pr head"
+    conditions:
+      # Jenkins CI failing
+      - status-failure~=continuous-integration/jenkins/pr-head
+    actions:
+      label:
+        add: 
+          - "needs: work"
+        remove: 
+          - "needs: review"
+
+  # From needs: review to needs: work - CI failure
+  - name: "label needs: work when Jenkins CI failed - any of the pipeline"
+    conditions:
+      # Jenkins CI failing - any of the pipeline
+      - status-failure~=^jenkins-ci
+    actions:
+      label:
+        add: 
+          - "needs: work"
+        remove: 
+          - "needs: review"
+
+  # From needs: review to needs: CI
+  - name: "label needs: CI when at least one reviewers approval"
+    conditions:
+      # Labels
+      - "label!=needs: work"
+      - "label!=needs: preceding PR"
+
+      # Reviewers
+      - "#changes-requested-reviews-by=0"
+      - "#approved-reviews-by>=1"
+
+      # CI green policy, at least Travis should be green
+      - status-success~=continuous-integration/travis-ci/pr
+      # new CI needs to be done
+      - status-neutral~=continuous-integration/jenkins/pr-head
+    actions:
+      label:
+        add: 
+          - "needs: CI"
+        remove: 
+          - "needs: review"
+
+  # Conflict in the PR - needs: work and a comment to notify a user
+  - name: "label needs: work when there is a conflict"
+    conditions:
+      - conflict
+    actions:
+      label:
+        add:
+          - "needs: work"
+        remove: 
+          - "needs: review"
+      comment:
+        message: This PR cannot be merged due to conflicts. Please rebase to resolve them.
+
+  - name: "add label feature branch for feature branch additions"
+    conditions:
+      - base~=^feature
+    actions:
+      label:
+        add:
+          - "release-version: feature branch"
+
+  # Ready for integration. Not yet auto merge, will be enabled once carefuly tested
+  - name: label "ready for merge" when ready
+    conditions:
+      # Labels
+      - label!=do not merge
+      - "label=needs: CI"
+
+      # Reviewers
+      - "#approved-reviews-by>=1"
+      - "#changes-requested-reviews-by=0"
+
+      # CI green policy
+      - status-success~=continuous-integration/travis-ci/pr
+      # Internal Jenkins - we rely on PR head to provide status
+      - status-success~=continuous-integration/jenkins/pr-head
+      # any of the jenkins pipeline needs to be green
+      - status-success~=^jenkins-ci
+    actions:
+      label:
+        add: 
+          - ready for merge
+        remove: 
+          - "needs: CI"
+
+  # Clean-up after merge
+  - name: remove ready for merge when merged
+    conditions:
+      - merged
+      - label=ready for merge
+    actions:
+      label:
+        remove:
+          - ready for merge
+
+  - name: add "do not merge" label when WIP is in title
+    conditions:
+       - title~=^(\[wip\]( |:) |\[WIP\]( |:) |wip( |:) |WIP( |:)).*
+    actions:
+      label:
+        add:
+          - do not merge
+
+  # Check if version label is applied
+  - name: release version is a must for merged PRs
+    conditions:
+      - merged
+      - -label~=^(release-version)
+    actions:
+      label:
+        add:
+          - Release review required
+      comment:
+        message: This PR does not contain release version label after merging.
+
+