Petitboot v1.10.4 tagged

[Samuel Mendoza-Jonas]

Petitboot v1.10.4 has been tagged, including a few fixes and quality of
life improvements:

- Bunch of minor fixes or updates to make scan-build happy
- Fixup to the pb-plugin script for name and vendor values
- Add a sudo promopt to pb-exec to support plugins
- Add & publish a first run at in-tree documentation
  (You can see this here! https://open-power.github.io/petitboot/)
- Two fixes to grub-parser around 'id' and default handling.