sstate: Add extra intercept functions

In some cases we do either need to add extra sstate manipulation
functions, or change the existing modification functions. This patch
parametrises them to SSTATECREATEFUNCS and SSTATEPOSTUNPACKFUNCS and
abstracts the "hardcoded path" functions into separate functions using
these new variables.

We may use this new functionality to improve binary relocating using
patchelf for example, this at least lets us have the hooks to be able to
experiment.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
patchelf: Add patchelf recipe

This is useful since it allows us to edit the RPATH and interpreter fields
in ELF binaries. This is potentially particularly useful in improving the way
we can use the SDK and also potentially making native binaries more relocatable.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>