yup, indeed you would need to build it...
hmm, did you install the required components , using platform_linux/build.sh
its really important you use the correct version of the compiler.
I had this running on a PI 2, so it will work
BUT...the performance is no where near enough to make the UI useable.
see this post:
have you tried running the UI yet? even though the firmware doesnt compile the UI should work....
frankly, Id try this first, as I doubt the odriod has enough cpu/gpu power to make the axoloti UI a pleasant experience ... though id be pleased to hear otherwise