####With Homebrew:####
brew update
brew upgrade
brew install binutils libusb
brew cask install gcc-arm-embedded # This is where you get the ARM cross-compiling stuff
sudo pip2 install pyusb
####Fix the environment statements:####
The scripts look for python2, but you likely just have that installed as (system) python. Hence:
sudo ln -s /usr/bin/python /usr/bin/python2
After this, make flash as expected.