Support for save/restore system clock from file

[troglobit]

Description

This PR mainly addresses issues with malfunctioning RTC devices. A few minor "infra" changes have also been included, listed at the end:

Major:

• Disable RTC on Styx boards, can return time in the future, and no battery backup
• Backport upstream Finit patches to support saving and restoring system clock from a file
• Enable Finit RTC fallback to disk file and set fixed RTC fallback value in all defconfigs
• Fix finit.mk handling of .config string values

Minor:

• mnt script, add missing -n to test
• Hide "use persistent /var ..." section in qemu.sh -c when disk image is selected
• Prevent disk image from being recreated every time qemu.sh is started
• Fix misaligned grub menu when booting disk image in qemu.sh
• Add support for tweaking emulated RTC behavior in qemu.sh

Checklist

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

• [X] Bugfix
  • [ ] Regression tests
  • [x] 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):

[troglobit]

@wkz pretesting Finit patches, I'll make sure to add a ChangeLog update and a better PR description tomorrow.

[troglobit]

@mattiaswal maybe you can focus your review on the non RTC-related changes? (If you're pressed for time, that is.)