UniRFRemix - Cleaned up error checking + Bug fix

[ESurge]

What's new

• Cleaned up error checking in unirfremix_cfg_set_check function.
• Fixed a bug where all options are disabled and set to N/A but it doesn't show Config is Invalid screen.
• Added a Checking Config screen which is the default screen while the app checks the .sub files (useful for slower SD cards)

Description of Fixed Bug

• Had a case where valid .sub files were being set as N/A
• When all buttons were assigned to these files, all labels were N/A and buttons were disabled but Config is Invalid screen did not show.

Verification

• Create a test file with errors and ensure checking is properly done
• If at least one button is valid, app should proceed normally with errored buttons disabled and labels set to N/A
• If all buttons have errors, app should show Config is Invalid screen

Notes

• Did not check the tx_not_allowed screen as I did not change any code regarding this check

Checklist (For Reviewer)

• PR has description of feature/bug
• Description contains actions to verify feature/bugfix
• I've built this code, uploaded it to the device and verified feature/bugfix

[xMasterX]

Thanks!