Added option for more physical buttons via I2C expander PCF8574

[tomasbubela]

First of all, thank you for your amazing Pendant FlidDial project – it's really well made!
I personally missed having more physical buttons, so I’ve modified the design to support multiple buttons.

Currently, there are 8 buttons connected via the PCF8574 I2C expander, which also allows for easy future extension by daisy-chaining more expanders.

The additional buttons are currently used for Multi Jog functionality.
Two out of the eight buttons are not yet assigned to any specific function, leaving room for further customization.

[MitchBradley]

I tested this and found the following problems:

1. When you compile the m5dial_more_buttons env, there are many compilation warnings saying that FNC_BAUD is redefined. I think that the first definition of -DFNC_BAUD needs to be moved from the m5_base env into the m5 env
2. If I run the m5dial_more_buttons build on a stock M5Dial without the I2C expander chip, it spews i2c errors on the debug console at a high rate. That makes the i2c build unacceptable for use on a standard M5 Dial.
3. Because of (2) and the FNC_BAUD rate change, an m5dial_more_buttons build cannot be the standard released version, so making it the default env is not good.

If you will fix these problems, I will accept that patch as an alternative that people can choose to build themselves. The i2c version could be the standard released version if you can make it so that I2C support does not cause problems on a stock M5 Dial without an expander chip, and if the baud rate is the same as standard.

[MitchBradley]

One more thing - the FluidNC wiki would have to be modified to add complete instructions for using the expander.