Vastly improved CardBoot.

[lb3361]

This pull request pushes a vastly improved version of CardBoot:

• loads system.gt1 about five times faster (~ 2 seconds instead of 10 seconds)
• follows the cluster chain when reading FAT32 directories
• initializes SDv1 and SDv2 cards correctly

It also renames Apps/CardTest as Apps/CardBoot to reflect the fact that this code is only used for booting from SD and no longer as an operating system trial. All files except the CardBoot related ones have been moved into a 'old' subdirectory for the record.

[at67]

I think Marcel would be really happy to see work progressing here, well done.

[lb3361]

Cardboot remains quite limited about what it can load.

• It cannot load anything into page zero without crashing
• It cannot load things into the visible screen buffer (it clears the screen!)
• It cannot load things into the screen buffers hole above 0x72a0.

The solution to load such programs is to make CardBoot load the browser https://github.com/lb3361/gigatron-os/tree/master/sys1 which will autoload any program named 'autoexec.gt1' if it exists.