Move graphics to the on-chip flash.

This makes life simpler on ATSAM4LS4 MCU, which has enough on-chip flash.
The alternative setting allowed for the smaller ATSAM4LS2 by requiring the graphics data to be placed into default.lay on the external flash.
Windows compatibility.

Windows didn't recognise the bootloader's disk volume because it was marked as non-removable.
Also, USB MSC interface names have been made more descriptive, as they are displayed to the user everywhere under Windows.