CLI build instructions incorrect

[MrSurly]

Per the README.me in the esp32/ directory:

In order to build this project, a copy of the Espressif IDF is required and it's
path must be specified via the ESP_IDF_PATH variable. See the Makefile for details.

It should be IDF_PATH instead of ESP_IDF_PATH, since Makefile copies IDF_PATH to ESP_IDF_PATH, but application.mk only uses IDF_PATH; this causes the call to esptool.py to fail because the path is wrong.

Also, this line:

make BOARD=LOPY -j5 TARGET=boot

should read

make BOARD=LOPY -j5 LORA_BAND=USE_BAND_868 TARGET=boot

And I'd recommend some text about selecting the correct band for your region.

I can submit a PR, if desired.