mender-convert: RaspiOS version 2021-01-11 does not work on Rpi4

We have tried to update the base RaspiOS image to version 2021-01-11 but on Raspberry Pi 4 the converted image does not boot.

Related PRs:

• https://github.com/mendersoftware/mender-convert/pull/289
• https://github.com/mendersoftware/mender-convert/pull/296
• (and cherry-picks in other branches)

We need to investigate independently, fix, test, and only then port to release branches

Acceptance criteria:

• mender-convert master + RaspiOS version 2021-01-11 works for Rpi4
• Fix is ported to release branches
• Images are updated in the mender-docs [holding off till the release] ref: https://github.com/mendersoftware/mender-docs/pull/1367#pullrequestreview-608631619