USBasp 2011.05.28. for Atmega328p 16 MHz

- add F_CPU parameter in Makefile328
- add F_CPU parameter in Makefile and use it as pre-define
- allow predefined F_CPU define in clock.h and
  calculate CLOCK_T_320us from given F_CPU frequency
- allow predefined F_CPU define in usbconfig.h and
  calculate USB_CFG_CLOCK_KHZ from given F_CPU frequency
- add precompiled atmega328 hex file for 16 MHz quartz