kbuild - separation of source and object trees

The new kbuild lets you build outside of the source tree:
export KBUILD_OBJTREE=/path/to/obj/tree
export KBUILD_SRCTREE_000=/src/tree
make -f $KBUILD_SRCTREE_000/Makefile

Why bother?
Build multiple configurations
Build multiple architectures
...all from the same source tree