Formalize mender-convert-integration-scripts repo as part of mender-convert project

Currently, generating the integration binaries (uboot, etc) are handled outside of the standard mender-convert repo. The basics have been captured here and tested on Beaglebone. This should be formalized and the other platforms added and tested as part of that repo.

Acceptance criteria:

• Repository mender-convert-integration-scripts in moved to mendersoftware
• A Pipeline is created which builds the integrations
• The resulting bootloader binaries are published in an S3 bucket
  • Bucket structure is something like mender/mender-convert/uboot/<device_type>/<config-name>-<uboot-version>.tar
• mender-convert is updated to use the new files from S3