Fix build error on empty root folder

If a build target does not contain system image, it doesn't
output anything to root folder. The patch fix the build error for
the case the root folder is empty.

Bug: 212486689
Test: make bootimage dist
Test: Build pass
Change-Id: I4f1376eb7e615f48171ed9c3057c5a78074abaa6

core/Makefile

@@ -784,6 +784,7 @@ INSTALLED_FILES_JSON_ROOT := $(INSTALLED_FILES_FILE_ROOT:.txt=.json)
 $(INSTALLED_FILES_FILE_ROOT): .KATI_IMPLICIT_OUTPUTS := $(INSTALLED_FILES_JSON_ROOT)
 $(INSTALLED_FILES_FILE_ROOT) : $(INTERNAL_ROOT_FILES) $(FILESLIST) $(FILESLIST_UTIL)
 	@echo Installed file list: $@
+	mkdir -p $(TARGET_ROOT_OUT)
 	mkdir -p $(dir $@)
 	rm -f $@
 	$(FILESLIST) $(TARGET_ROOT_OUT) > $(@:.txt=.json)