negative offsets are not supported

[McR42]

On Toradex modules, the uboot-env is put last in the MMC boot sector.
That way offset has to be counted from the end of the sector.
This is achieved by using a negative offset.

Switching u-boot-tools to libubootenv-tool breaks this configuration, because libubootenv does not (yet) support negative offsets.
There is a patch that provides negative offsets, but it's from April 2020 and therefore seems not included in latest libubootenv0.1 (0.2-1) (which dates to January 2020, as far as I understand the changelog).

Please update, so we get back functionality we depend on.

In case there is doubt about versioning,
We are on a recent Ubuntu 20.04LTS, with
- u-boot-tools (2021.01+dfsg-3ubuntu0~20.04-4)
- libubootenv-tool (0.2-1)
- libubootenv0.1 (0.2-1)

Thanks.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.