implemented ability to enable -D_FF_RV_EXIST_ in TIF library It is not clear if this quirk is intentional or not, but TI's production version of tif_na7_db_{fl,ir}.lib was built without -D_FF_RV_EXIST_, and as a result, the code that is present in the source for the TRACEMASK_IN_FFS feature was not compiled in, making the feature unavailable. The present change makes it possible to re-enable this code (when TIF libraries are being rebuilt from source rather than used as blobs) by setting TRACEMASK_IN_FFS=1 on the configure line. The new code is NOT enabled by default: it is not a change which we can justify inflicting on our production firmwares.