BitBake will build packages for the target machine selected in the MACHINE variable inside the local build settings (conf/local.conf).

Looks like there's already a meta-nanopi layer providing support for the NanoPi board, so (in theory) building for NanoPi should be pretty straightforward.

As soon as meta-eosio is ready I'll make a new article with a guide on how to build and install GNU/Linux packages and images for popular arm boards. I'll try to make sure the guide is generic so NanoPi is also covered :)