Compile issues with recent ESPHome versions fixed:

  - An error about toolchain-riscv32-esp not being found (the PlatformIO tool manager
    tries to install this one, even though the lamp is an ESP32 device and not riscv).
  - On systems for which the above toolchain could be found, compilation would fail
    with an error about `esp_mac.h` not being found.

Translation provided by: @Thisisnotgoingpublished
See: https://github.com/mmakaay/esphome-xiaomi_bslamp2/issues/88

Thanks to @kroimon for the heads up!

The current version of ESPHome shows a warning about GPIO4:
```
WARNING GPIO4 is a Strapping PIN and should be avoided.
Attaching external pullup/down resistors to strapping pins can cause unexpected failures.
See https://esphome.io/guides/faq.html#why-am-i-getting-a-warning-about-strapping-pins
```
The warning can be ignored, so I added a remark about this to the documentation.
Thanks to @jossuanet for the heads up about this warning.

Same fix as already applied for release/2021.10.0b1.