allow esptool.py invocation to be set via make vars
allow esptool.py invocation to be set via make vars

This commit replaces the literal calls to `esptool.py` with the
`$(ESPTOOL)` Makefile variable. This allows one to set the esptool
invocation on the Make command line:

    make ESPTOOL="python2 $(which esptool.py)"

(or via the environment, an include file, etc)

Closes #793

adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/micropython,adafruit/micropython