netkvm: avoid rebuilding the driver each time
Do not update netkvmmof.h without need, check whether
generated header file is different from existing one,
only then update it.
mofcomp.exe -B:.\netkvm.bmf ..\Common\netkvm.mof
-wmimofck.exe -h..\Common\netkvmmof.h -m -u .\netkvm.bmf
+wmimofck.exe -h..\Common\tmpmof.h -m -u .\netkvm.bmf
+fc ..\Common\netkvmmof.h ..\Common\tmpmof.h > nul
+if not errorlevel 1 goto thesame
+echo Updating netkvmmof.h
+copy /Y ..\Common\tmpmof.h ..\Common\netkvmmof.h
+:thesame
+del ..\Common\tmpmof.h
