Add way to get F_CPU at runtime. Update Serial.setBaud to work at runtime

* If YAAL_UNSTABLE_F_CPU is set, then cpu.clock.get() will return F_CPU at time of call.
  If that macro is not set, it returns compile time F_CPU
  This feature to work correctly at least one of F_CLOCK, F_CPU is needed. You should provide F_CLOCK as crystal speed and F_CPU as initial clock speed.

* Serial has new setBaud method, which should be very fast also when cpy.clock is dynamic (runtime) and/or baud rate is not combile time constant.