Migrate features from seewood fork (rev 2)

[yo1dog]

As requested, I have migrated changes from the seewood fork to the base repo:

• Custom DOL loading based on held button.
• All 4 controllers supported.
• Hold dpad-down to delay exiting (allowing users to read diagnostic messages).

Makefile and README changes will be submitted in a separate pull request.

[yo1dog]

Tested on my DOL-101 via PicoBoot.

[9ary]

I would prefer if you'd update the previous pr instead of opening a new one. You can force push the branch to do that.

[yo1dog]

Closing in favor of #7.