Project-specific settings (like which board you’re using) are outlined in this brief Makefile that then includes the larger Makefile which contains the actual nuts and bolts for building your code.

Kinda new at makefiles, and I've been struggling with this problem for a while now. How do I get makefile to work with just a specific subset of files located in a source folder?

And we never have to re-compile the library code again. In this version of the makefile, OBJS is just the three local object files. Everything else is in the core.a archive.