Possibility to define pin aliases

**Describe the problem you have/What new integration you would like**

All of my devices include a basic yaml with general configuration, like `api`, `ota`, etc. This configuration also includes `status_led` with `pin: LED`. This works fine with commercial boards which have a LED alias defined, but it doesn't work for custom boards.

I'd like to be able to define custom pin aliases, something like this:

```
esp8266:
board: esp8285
pin_aliases:
LED: 13
```

**Please describe your use case for this integration and alternatives you've tried:**

I can't reuse my common yaml templates unless the board has a defined LED alias already.

**Additional context**

This might also be solved by something like https://github.com/esphome/feature-requests/issues/2348, but this solution might be simpler.

No implementation files or tests are named. Start by tracing how board-defined pin aliases are loaded and resolved, then compare the proposed `pin_aliases` YAML with feature-requests/issues/2348. Done means custom aliases such as LED: 13 can be defined and reused by common YAML templates on custom boards.

Written by the indexing model from the issue text.