In order to construct an SDK containing the build system we will need a way to construct the sysroot and other parts that are already managed by sstate, but outside of the context of a build. We will have the signature and the corresponding sstate archive; we need to extract that and then write a stamp file so that the build system knows the task need not be executed. This is basically doing exactly what the sstate code does, except without going through an actual separate bitbake build in order to do it. The relocation that normally needs to be done upon extracting the sstate archive will also need to be deferred until SDK installation (i.e. stored away somewhere so it can be run when the SDK is installed).
Perhaps it isn't already clear, but the objective of this is to have the sstate packages already extracted in the SDK rather than having to do all of that as a step within the installation. If we must actually engage bitbake to do this then we can, but the important thing is that the relocation / postinstall scripts must still be run at SDK installation time or the binaries won't be properly prepared to work in the location the SDK has been installed.
I believe the extensible SDK already covers this usecase now and we have more specific bugs related to that for future development.