You can use any of the following:
make package/foo-clean (wipes out the ipkg and the directory in build_mipsel)
make package/foo-mostlyclean (performs a "make clean" in the package's build_mipsel)
make package/foo-prepare (downloads the source, unpacks to build_mipsel and patches)
make package/foo-compile (self explaintory)
make package/foo-install (produces the ipkg file in bin/packages)
make package/foo-rebuild (alias for "mostlyclean, compile")
The above steps also produce files in the package's directory
.prepared
.configured
.built
You can manually remove any one of those and run make which will force it to perform that step again. And yes, I know the build system is an undocumented mess -- the buildroot-ng rewrite should solve the issues.