Add support for overriding the compatible string

[troglobit]

Description

Some customers would like to seamlessly upgrade between their branded Infix-based software and vanilla Infix. E.g., for testing of new features and end-customer support.

This PR adds support for overriding the default compatible string, which by default is taken from INFIX_IMAGE_ID, created from the INFIX_ID in the defconfig and BR2_ARCH. This new setting, INFIX_COMPATIBLE is by default INFIX_IMAGE_ID.

Background, the INFIX_IMAGE_ID is also used for the image file names, as well as the tarballs, so overriding that in the defconfig has wider implications. Hence this extension.

Checklist

Tick relevant boxes, this PR is-a or has-a:

• Bugfix
  • Regression tests
  • ChangeLog updates (for next release)
• Feature
  • YANG model change => revision updated?
  • Regression tests added?
  • ChangeLog updates (for next release)
  • Documentation added?
• Test changes
  • Checked in changed Readme.adoc (make test-spec)
  • Added new test to group Readme.adoc and yaml file
• Code style update (formatting, renaming)
• Refactoring (please detail in commit messages)
• Build related changes
• Documentation content changes
  • ChangeLog updated (for major changes)
• Other (please describe):

[wkz]

LGTM 👍