Updating the LED0 state from systick handler ensures LED0 is always
consistent with its flash rate regardless of other processing going on in
either interrupts or main. This improves the visible stability of the
bootloader, rather than LED0 flashing somewhat randomly at times.
This commit also changes the LED0 flash rate depending on the current state
of DFU, giving slightly more visual feedback on what the device is doing.
9dd470b768