Changes for Step8

Driver for USB keyboards (class CUSBKeyboard) added
Detects low- and full-speed devices
class CKeyMap added
class CDWHCIFrameSchedulerNonPeriodic added
class CDWHCIFrameSchedulerPeriodic added
FRAME_BUFFER_SIZE for Ethernet is now 1600 (a bit too much but to be sure)
Timer interrupt does not invert audio lines anymore