Reorganize MCU board support examples into modules

[task-jp]

Move MCU board support example files into their own directories as mod.rs files.

[tronical]

Hmmm. I think the majority of modules in our repo don't use subdir/mod.rs. Is there a specific future change that you have in mind that benefits from this?

[task-jp]

There is one source code and one directory for each board support, but IDE and file managers display directories and files separately (directories come first), so the two are separated, which is inconvenient.

If the source code could be moved under the directory, it would be much easier to work on board support tasks by working only under that directory. There is no strong intention to unify using mod.rs, so keeping the source file names as they are is fine.

Additionally, since it is anticipated that more board supports will be added in the future (right?), having the mcu-board-support directory clean and organised would be preferable.

[tronical]

There is no strong intention to unify using mod.rs, so keeping the source file names as they are is fine.

Unless there's a good reason to handle things differently in this sub-directory, I'd prefer to keep a consistent way of handling modules across the entire repository then and not use mod.rs.

[task-jp]

I kept file names as they were.