Remove drive->desc field.

Remove drive->desc field.

The description field is only available during the POST phase - it is
confusing to have it live in a structure available through all phases.

The description was only used by the boot menu code - pass each drive
description directly to the bootlist code.

Add a helper (znprintf) to automatically malloc_tmp the required
space.

Also, fixup ramdisk handling - it was using an incorrect floppy
priority.