`ztimer_core` functions have to reside in IRAM for timing reasons and to be available also when the IROM cache is disabled. Although the module is called `ztimer`, its object files are generated in directory `ztimer_core`.
The functions of the `esp*/freetos` libraries must be placed in IRAM because they can be called when the IROM cache is disabled. While the functions implemented in `cpu/esp8266/freetos/*.c` are already placed in IRAM, the functions implemented in `cpu/esp_common/freetos/*.c` are not placed in IRAM. The reason for this is that the object files of these files are created in the `esp_freertos_common` directory, which is not included in the `esp.riot-os.ld` file because the library is named `esp_freertos_common`.
