cpu/esp32: FIX use esptool.py from path

This allows flashing using `esptool.py` install from pip.

The documentation already state that `esptool.py` must be put available
in the path and that it is possible to use the `pip` version.

cpu/esp32/Makefile.include

@@ -64,7 +64,7 @@ PSEUDOMODULES += esp_spiffs
 PSEUDOMODULES += esp_wifi_any
 
 export TARGET_ARCH ?= xtensa-esp32-elf
-export ESPTOOL ?= $(ESP32_SDK_DIR)/components/esptool_py/esptool/esptool.py
+export ESPTOOL ?= esptool.py
 
 USEMODULE += esp_idf
 USEMODULE += esp_idf_driver