Resurrect the backup memory port

We need this to work for our chip, and it's not been tested in a long
time in upstream -- it didn't even used to build since the Nasti
conversion.  This makes a few changes:

 * Rather than calling the backup memory port parameters MEM_*, it calls
   them MIF_* (to match the MIT* paramater objects).  A new name was
   necessary because the Nasti stuff is now dumped as MEM_*, which has
   similar names but incompatible values.

 * p(MIFDataBits) was changed back to 128, as otherwise the backup
   memory port doesn't work (it only send half a TileLink transaction).
   64 also causes readmemh to bail out, but changing the elf2hex parameters
   works around that.

 * A configuration was added that enabled the backup memory port in the
   tester.  While this is kind of an awkward way to do it, I want to
   make sure I can start testing this regularly and this makes it easy to
   integrate.

vsim/Makefrag

@@ -67,8 +67,6 @@ $(simv_debug) : $(sim_vsrcs) $(sim_csrcs) $(sim_dir)/libdramsim.a $(consts_heade
 	$(VCS) $(VCS_OPTS) -o $(simv_debug) \
 	+define+DEBUG -debug_pp \
 
-#	+define+MEM_BACKUP_EN \
-
 #--------------------------------------------------------------------
 # Run
 #--------------------------------------------------------------------