[llvm-objcopy] Change --only-keep to --only-section

I just hard core goofed when I wrote this and created a different name
for no good reason. I'm failry aware of most "fresh" users of llvm-objcopy
(that is, users which are not using it as a drop in replacement for GNU
objcopy) and can say that only "-j" is being used by such people so this
patch should strictly increase compatibility and not remove it.

Differential Revision: https://reviews.llvm.org/D52180

llvm-svn: 348446
diff --git a/llvm/tools/llvm-objcopy/CopyConfig.cpp b/llvm/tools/llvm-objcopy/CopyConfig.cpp
index 7361177..24f0e29 100644
--- a/llvm/tools/llvm-objcopy/CopyConfig.cpp
+++ b/llvm/tools/llvm-objcopy/CopyConfig.cpp
@@ -315,8 +315,8 @@
     Config.ToRemove.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_keep_section))
     Config.KeepSection.push_back(Arg->getValue());
-  for (auto Arg : InputArgs.filtered(OBJCOPY_only_keep))
-    Config.OnlyKeep.push_back(Arg->getValue());
+  for (auto Arg : InputArgs.filtered(OBJCOPY_only_section))
+    Config.OnlySection.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_add_section))
     Config.AddSection.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_dump_section))