A crystax ndk based Android build system for bx is available here:
https://github.com/crystax/libbitcoin-explorer

This is the git diff between it and the current bx master branch as of
commit 19324e0881279f99aa6214ad030ba06300fba9fe: http://ix.io/15O8

The build supports aarch64 only, but it could be made to support other
architectures.