Baud rate setting in boot8266 is not ideal

[pfalcon]

Fix from #2 reads:

    // At this point, hardware doesn't yet know that it runs with 26MHz
    // crystal oscillator instead of "default" 40MHz, so adjust baud rate
    // accordingly.
    uart_div_modify(UART0, UART_CLK_FREQ / (BAUD_RATE * 40 / 26));

This is however appears to be true only for reset/powerup. When rebooted using machine.reset(), crystal is apparently remains configured as 26MHzm so baud rate is off. So, need to check actual system bus speed.