Implement auto config of emonTx CM FW to enable serial mode

[glynhudson]

https://community.openenergymonitor.org/t/no-data-and-cannot-connect-to-emoncms-on-emonpi-with-emonesp/15062/14

Serial command procedure to disable RF and enable emonESP serial:

+++
w0
s
x

https://github.com/openenergymonitor/EmonTxV3CM#serial-configuration

[jeremypoulter]

I was thinking about this. the biggest problem is you need a way for the EmonESP to reboot the EmonTX, so either an additional command on the EmonTx firmware, and/or connect a GPIO on the ESP to the reset line.

[danbates2]

I desired this recently also, for the Tx calibration rig I just made. It'd be worth pitching the idea to Robert.