Improve EfiDiagnostics flush code

[jstarks]

Please see comments from #1677 (comment)

[maheeraeron]

This should now be addressed as part of these two PRs:

• firmware_uefi: EfiDiagnostics switch to polling instead of Arc<Mutex<>> #1693
• firmware_uefi: EfiDiagnostics change to tracelimit with custom period and limit #1706