Support query-able minimum version support

[fhunleth]

For intermediate upgrade scenarios, i.e., those where a device needs to upgrade to an intermediate version of software before upgrading to the final version, it is desirable to be able to extract the upgrade path from the .fw file. The current situation is that you can manually set this scenario up on a firmware update server, but it would be less error prone if the .fw file metadata could help out.

One idea: Add a meta-min-version that can be queried using fwup -m and support a require condition to run a semver check on a u-boot environment block variable (similar to require-uboot-variable).