Implement support for broadcast

[T0RAT0RA]

Hi,

I added support for broadcast messages.

Documentation can be found here: Modbus_over_serial_line_V1.pdf (see page 7/44)

In broadcast mode, the master can send a request to all slaves. No response is returned to broadcast requests sent by the master. The broadcast requests are necessarily writing commands. All devices must accept the broadcast for writing function. The address 0 is reserved to identify a broadcast exchange

[T0RAT0RA]

Hey @brainelectronics, do you think you could have another look at the PR? :)

Thanks!