Support loading selfboot folders as if they were a CDI

[StrikerTheHedgefox]

Is your feature request related to a problem? Please describe.
I'm currently contributing to a DC homebrew project, and every time I want to test it with Flycast, I need to build it into a new CDI file first, for every single test run. This can take up a lot of time when iterating and testing, and also easily build up to many gigabytes written in a day, wearing down my SSD's lifespan.

Describe the solution you'd like
I'd like to have the ability to load a loose selfboot folder as if it were a CDI, for ease of development/debugging.

Describe alternatives you've considered
I've tried loading an elf file directly, but it doesn't see the accompanying files. I've tried other emulators, nothing works. dcload-ip is a no-go, seems to be broken.

Additional context
Image showing the contents of the folder, I'd like to be able to load this as if it were a CDI image.