Improve the release note extraction

Author: fhunleth

.circleci/config.yml

@@ -284,7 +284,7 @@ jobs:
           key: nerves/deploy/{{ checksum ".target" }}-{{ .Revision }}-{{ .Environment.CIRCLE_TAG }}
       - run:
           name: Create release notes - FAILS if tag isn't in CHANGELOG.md
-          command: grep -Pazo "(?s)(?<=## ${CIRCLE_TAG})[^#]+" ~/deploy/CHANGELOG.md | sed '/./,$!d' > ~/deploy/RELEASE_NOTES
+          command: awk -v tag="## ${CIRCLE_TAG}" 'BEGIN {found=0} $0 ~ tag {found=1; next} found && /^##/ {found=0} found {print}' ~/deploy/CHANGELOG.md | sed '/./,$!d' > ~/deploy/RELEASE_NOTES
       - store_artifacts:
           path: ~/deploy
           destination: images