PSP Overlays Created by `make-config.py` Do Not Follow Current Patterns

[hohle]

When creating new stage overlays for PSP, the current convention is to set the source path to src/st and prefix segments with either the bare overlay directory for shared code or ${ovl}_psp for PSP specific code. The make-config.py generated splat config, however, sets the source directory to src/st/${ovl} and creates a subdirectory named ${ovl}_psp within that directory for PSP specific code.

make-config.py should be updated to create files with the current convention.

RE: #2936