I am trying to figure out how to first repoduce your build, then try to upgrade FFOS to 2.1.
Is that repo the entry point I should start with?
Yes, that's the place to start.
Instructions there are missing the part of applying patches from the patches directory (I was too lazy to fork every project over a single patch). Copy each patch into to the project directory of a same name (e.g. frameworks/av), change into directory and apply with
'make' will create fastboot flashable images. If you want to create recovery flashable packages ('make otapackage'), you need to copy signapk.jar into out/host/linux-x86/framework/ first. There's no java in B2G build system to compile it from the source.
Also updating to the very latest CAF tag broke translations. There's few options:
1) Use the previous tag that was used to make the build in the OP.
2) Reset HEAD in compare-locales to commit 1c880a1258b7d96641010c90ed9b9bd0cc43b39d
3) Revert this commit
so extra languages are not included in the build