Fix rm function on Windows once more.

Windows del doesn't handle a non-existing folder gracefully, so check for
existence first.

Change-Id: I9c401f3f3a3e1e01b80d29e335256fbf3db22d35
diff --git a/rbutil/libtools.make b/rbutil/libtools.make
index 0be3fe9..4623808 100644
--- a/rbutil/libtools.make
+++ b/rbutil/libtools.make
@@ -23,7 +23,7 @@
 TOP := $(dir $(lastword $(MAKEFILE_LIST)))
 ifeq ($(OS),Windows_NT)
 mkdir = if not exist $(subst /,\,$(1)) mkdir $(subst /,\,$(1))
-rm = del /q /s $(subst /,\,$(1))
+rm = if exist $(subst /,\,$(1)) del /q /s $(subst /,\,$(1))
 else
 mkdir = mkdir -p $(1)
 rm = rm -rf $(1)