[oe] kernel.bbclass: Fix do_shared_workdir task ordering

[S. Lockwood-Childs]

http://patchwork.openembedded.org/patch/99875/

Apparently this patch is still not in master, and I just ran across the
problem with an externally built module (omaplfb from omap3-sgx-modules) 
in meta-ti layer.

What's the plan for getting a correct Module.symver into shared_workdir 
for external modules to build against? Above patch, or does someone have
an even better idea?