Image set selection

[erichelgeson]

Allow for BlueSCSI to load image sets without having to move files around on the SD card. As an example this will allow users to use the same BlueSCSI and switch between their Mac Plus, PowerMac, PowerBook, etc without having to remove the SD card and change/move files.

Using Pins A1/B1 and the internal pullup, check at startup and load the appropriate ImageSetN folder. Default is to load the root of the SD card so there is no breaking behavior.

If the optional /ImageSetAll directory exists - it will always load images in there. This way you can have a shared drive between all sets.

If an ID conflicts, the last one wins. Order is ImageSetN, then ImageSetAll.

A1	B1	#
H	H	default / or /ImageSet0 if exists
L	H	/ImageSet1
H	L	/ImageSet2
L	L	/ImageSet3

Questions:

• Does ImageSet make wording make sense for users?
• We're low on Pins - I chose two - any better choices?

Hardware:

You can add this to any BlueSCSI 1.0-a -> 1.1-a with PA1, BA1, and Ground to an 3x2 block headers. Future versions of BlueSCSI will have header or jumpers (or both) on the board.