Mac OS X sends a SCSI command to remove the medium when it unmounts a
drive. If this command is not honoured, then OS X will automatically
remount the drive, making it impossible to eject. This patch disables
the USB MSC when the right SCSI command is sent.
Can now choose at boot up whether the USB device is CDC+MSC or CDC+HID.
Choice is made by an option in boot.py, with default being CDC+MSC.
HID+MSC is not currently supported, but should be easy to implement.
Boot up now has ability to change the reset mode: hold down USR switch
while booting and LEDs will count from 1 to 7 to indicate the boot mode.
Release USR when correct mode is selected. Current modes are 1 (normal
boot), 2 (safe mode), 3 (reset FS mode).
