SlugOS: preferred-slugos-versions.inc - fix kernel SRCREV selection

SlugOS: preferred-slugos-versions.inc - fix kernel SRCREV selection
This makes the PREFERRED_VERSION selection for the SlugOS kernel work
again after a recent "pretty-ification" change for SRCREV handling.
Note that this is a sub-optimal solution, but it is required to correct
build failures for SlugOS.  A long-term solution needs to either put
the SRCREV back to where it was (a semi-global config file), or it needs
to change the way the ixp4xx kernels are selected.  More specifically,
the problems this fixes for SlugOS still exist in the ixp4xx machine
definition file.