To my future self: this is how you can ‘burn’ an ISO file into a flashdrive, so that it’s bootable!
hdiutil convert -format UDRW -o destination.img kali-linux-2017.2-amd64.iso
diskutil list
diskutil partitionDisk /dev/disk2 1 "Free Space" "unused" "100%"
sudo dd if=destination.img.dmg of=/dev/disk2 bs=1m
diskutil eject /dev/disk2