Goal: Reduce build time when starting with a cleaned-up source tree. Useful when using an automated build system like Hudson/Tinderbox.
Just do the same as everytime you (re)compile OpenWrt. Checkout svn/git, set your options in .config and build the whole thing. Only difference this time: you call the directory toolchain.
We start at the same point as the toolchain. Checkout OpenWrt into directory openwrt (or anything else than toolchain). Now we need some patching, because at the moment (rev 27109) it is not possible to use OpenWrt for both.
Apply these patches in the order listet here:
After this, copy the .config-file from the toolchain directory into this build directory and do a make menuconfig.
Save and compile. For my case, the full toolchain build now needs about 1 1/2 hour and the normal firmware takes only 1 hour.