Makefile: rework the perversion that dynamically creates rules
Makefile: rework the perversion that dynamically creates rules

Segregate the object files that comprise different libraries into
(potentially) different subdirectories of the board work area; also
preserve the relative location of the source file in the object file
name, to reduce the likelihood of naming collisions.  This will allow us
to more easily support multiple bspacm libraries.

pabigot/bspacm,pabigot/bspacm,pabigot/bspacm,pabigot/bspacm