Consolidate the programmer options to be more logical and flexible

The configuration of programmers and use of them is inconsistent in the UI and config files 1.5.6. For example :

Preferences file upload.using option - to set bootloader vs others. Why is bootloader not just another programmer type under tools, programmer ?, then preference could be removed and consistency provided

This would then allow the upload and upload using programmer to be consolidated to just upload - with the type set by the tools programmer option, which would also allow the upload button on the UI to work across both types of programming.

There are also two bugs in the programing tool executable selection from boards.txt. If you set another programmer other than avrdude, even when the new executable is in the same folder as avrdudem it fails to spawn that process.
Similarly, if you use tools burn bootloader in the same condition, it claims the file is not present when it is.

Start by tracing the Preferences file upload.using option, the Tools programmer setting, and the upload, upload using programmer, and burn-bootloader flows described in the issue. Inspect boards.txt executable selection and compare the reported process-spawn failures; done means the programmer options are consistent and the configured executables launch correctly across these paths.

Written by the indexing model from the issue text.