mender-convert v2: automate upload of grub-efi binaries

mender-convert v2 relies on having access to the following pre-built binaries:

• grub-efi (arm, aarch64, x86_64)
• grub-editenv (arm, aarch64, x86_64)

Currently hosted at,

https://s3.console.aws.amazon.com/s3/buckets/mender/mender-convert/grub-efi/?region=eu-north-1&tab=overview

(These where generated using Yocto on thud branch and manually uploaded.)

For the automated solution, however, we want to generate them without Yocto, extending the functionality of mender-convert-integration-scripts.

Acceptance criteria:

• Extend mender-convert-integration-scripts to build grub-efi and grub-editenv binaries (NOTE: These are board independent so we need only one for the three architectures).
• The binaries for the three architectures are uploaded to S3:
  • Bucket structure is something like mender/integration/grub/<arch>/<version?>/grub-efi, grub-editenv
• mender-convert uses these