Use the Brain GCS. Go into the firmware uploader and click 'rescue', and then plug in the brain board. See if it recognizes the bootloader. If so, flash it, and then try booting.
One other dumb thing to make sure of: There are charge-only microusb cables out there without the data lines connected. I have one in my junk drawer that causes 10 minutes of confusion every time I pull it out, because it'll power a Tau board but I can't do anything useful with it.