Store a copy of installed-files.txt in DIST_DIR if system.img failed to build

Because we already have rule to store installed-files.txt in DIST_DIR,
to avoid the slim chance of race condition, this copy is named as
installed-files-rescued.txt

Change-Id: I9fa6d8aded0ad5de1f1422d70a4224b3fc308e75
diff --git a/core/Makefile b/core/Makefile
index 7b3dfe4..a8be582 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -944,6 +944,7 @@
       || ( echo "Out of space? the tree size of $(TARGET_OUT) is (MB): " 1>&2 ;\
            du -sm $(TARGET_OUT) 1>&2;\
            echo "The max is $$(( $(BOARD_SYSTEMIMAGE_PARTITION_SIZE) / 1048576 )) MB." 1>&2 ;\
+           mkdir -p $(DIST_DIR); cp $(INSTALLED_FILES_FILE) $(DIST_DIR)/installed-files-rescued.txt; \
            exit 1 )
 endef