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

@@ -169,7 +169,7 @@ ifneq (,$(filter esp_cxx,$(USEMODULE)))
     UNDEF += $(BINDIR)/esp_cxx/cxa_guard.o
 endif
 
-ESPTOOL ?= $(RIOTBASE)/dist/tools/esptool/esptool.py
+ESPTOOL ?= esptool.py
 
 # The ELFFILE is the base one used for flashing
 FLASHFILE ?= $(ELFFILE)