unfortunately you have no choice but to rebuild the (patched) source-tree with the -bb option, ie you have to redo the -bp stage.
There is a --short-circuit option to rpmbuild but it doesn't work with -bb or -ba, so not very useful in this context.
That's why the rpmbuild method seems such an ordeal, you have to build the source-tree with -bp just so you can run 'make menuconfig' to create your custom .config, then fiddle with that and copy it back to SOURCES, and then rebuild the whole source-tree again for the actual compile with -bb.
(Before running the final 'rpmbuild -bb' you should type 'cd', otherwise you'll get a cwd error, since the kernel build directory is overwritten)
If you are not changing the .config from the default, but just patching some sourcecode, then after editing the kernel.spec file and placing the patch in SOURCES you can run rpmbuild -bb.