core: Redefine flags as enum

[gaaf]

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)
• Cleanup

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

Redefine some flags as enum instead of separate define. This makes it easier to see what flags are accepted by functions and with the right compiler flags, the compiler will warn when the wrong flags are used.

No functional changes are intended.

[henningw]

Thank you, looks good to me. Lets wait for other people to review as well before merging it.

[miconda]

OK for me.

[henningw]

Thanks, merged.