fix button-detection so screenshots and charge-only mode work without enabling UMS
firewire detection is now handled separately from usb detection
increase the usb thread priority while an UMS connection is active
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@16435 a1c6a512-1295-4272-9138-f99709370657
Still not enabled by default, #define USE_ROCKBOX_USB is still required to enable it.
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@16360 a1c6a512-1295-4272-9138-f99709370657