Bugfix SPI-Clock too high for 16 MHz firmware

- bugfix for SPI-Clock selection with F_CPU higher than 12MHz in isp.c
  This prevent manually decreasing baudrate for the 16MHz firmware,
  if ATmega devices with slow clockrate need to be programmed.
- add common Makefile to build firmware for different CPUs and Clocks
- add 16 MHz support for ATmega8
- exchange precompiled hex files