mender-convert to build/publish a Mender enabled Raspbian image

See epic for background details.

Acceptance criteria:

• mender-convert Pipeline converts a dual-fs image from Raspbian, with Mender installed but not configured.
• Resulting image should have at least 500MB free space in fs partition. If reasonable, use leave 1GB
• Resulting image is compressed with xz (LZMA) to optimize on size
• The resulting sdmg is published in a versioned manner into an S3 bucket:
  • s3://mender-convert-images/RASPBIAN-VERSION-NAME/ARCH/RASPBIAN-VERSION-NAME-mender-MENDER_VERSION.img.xz
• It is build/published in mender-convert Pipeline for master (using latest Mender master deb package), and on mender-convert tags and release branches (which will use a tagged Mender deb package).
• Stretch goal: extend the build/publish to the rest of the images that we support atm