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
This caused a fatal compiler diagnostic after #750. This compiler
flag is already specified in the atmel-samd builds, so it makes
sense to do it here for the same reasons.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
