sim: mips: namespace igen configure vars

To prepare moving this logic to the top-level configure, the vars
need to be namespaced.  Do that here to make it easier to review.
Basically sim_xxx -> SIM_MIPS_XXX when a var is exported from the
configure script to the Makefile, and sim_xxx -> sim_mips_xxx when
the var is internal in the configure script.
3 files changed