Fix rm function on Windows.

Windows del doesn't handle paths with / as separators, need to convert them
first.

Change-Id: Idb3efa96167aec5666517feabab67385ebd2db1a
diff --git a/rbutil/libtools.make b/rbutil/libtools.make
index 87a0c15..0be3fe9 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 $(1)
+rm = del /q /s $(subst /,\,$(1))
 else
 mkdir = mkdir -p $(1)
 rm = rm -rf $(1)