Environment variables on ESP32 chip?

Is there any way of setting an environment variable, or anything like an environment variable, when building an image for my ESP32 chip?

I know that I could bake my parameter into my program using a #define, but I would like to avoid rebuilding my whole program every time I change the value of this parameter.

Environment variables aren't really a thing for the ESP32 as it's a single "process" environment, but you can store configuration separately from your program.

How you do this depends on your programming environment. You tagged esp-idf so I'll assume that's what you're programming in.

ESP-IDF includes a library called "NVS" for "Non-Volatile Storage". NVS uses a partition of the flash storage to store key-value pairs. It's designed to store many small values - more like integers and short strings (up to 4000 bytes) rather than photos or videos or files. So it's suitable for things like WiFi configuration or security certificates.

ESP-IDF includes examples of how to use NVS.

A very simple example of reading from it would include code fragments that would look like this:

#include "nvs_flash.h"
#include "nvs.h"

// initialize NVS
esp_err_t err = nvs_flash_init();
if (err == ESP_ERR_NVS_NO_FREE_PAGES || err == ESP_ERR_NVS_NEW_VERSION_FOUND) {
    // NVS partition was truncated and needs to be erased
    // Retry nvs_flash_init
    ESP_ERROR_CHECK(nvs_flash_erase());
    err = nvs_flash_init();
}

// open it to start using it
nvs_handle_t my_handle;
err = nvs_open("storage", NVS_READWRITE, &my_handle);
if (err != ESP_OK) {
    printf("Error (%s) opening NVS handle!\n", esp_err_to_name(err));
} else {
    // read a string - will set ssid_len to the number of bytes read
    size_t ssid_len = SSID_MAX_LEN;
    char ssid[SSID_MAX_LEN];

    err = nvs_get_str(my_handle, "ssid", ssid, &ssid_len);
    if (err == ESP_OK) {
        printf("got WiFi SSID %s\n", ssid);
    } else {
        printf("NVS lookup failed");
    }
}

You can use the "NVS Partition Generator" utility to build an NVS partition image which you can then flash to the ESP32 independently of the firmware.

Because NVS uses its own partition separate from where the firmware is stored, data stored in NVS will survive firmware updates or changes (as long as the new firmware doesn't change the flash partition scheme) - even if the new firmware is a completely different application.