Official CI server for Tensorflow has some nightly builds with python wheels for raspberry pi armv7l, it is not officially supported by tensorflow yet, they officially support only 64-bit architectures so far, but I managed to get yolo-keras working on "orange pi pc plus" using their nightly build wheel file.
You can also find the scripts they used for building the wheel (actually it's cross-built using a docker container) in directory tensorflow/tensorflow/tools/ci_build
inside source code.
Some people also provided guides for native building, but it generally requires more effort to get it to work.
I suggest you start by trying the python wheel file for tensorflow v1.8.0 for raspberry pi armv7l architecture, found here.