Fix dependency on BOARD_AVB_SYSTEM_KEY_PATH

Bug: 130111713
Test: treehugger
Change-Id: I2183ac9f8c45eec178876768dfcc83ae62445293
diff --git a/core/Makefile b/core/Makefile
index ed4c06a..79fd0ef 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -2230,6 +2230,9 @@
            exit 1 )
 endef
 
+ifeq ($(BOARD_AVB_ENABLE),true)
+$(BUILT_SYSTEMIMAGE): $(BOARD_AVB_SYSTEM_KEY_PATH)
+endif
 $(BUILT_SYSTEMIMAGE): $(FULL_SYSTEMIMAGE_DEPS) $(INSTALLED_FILES_FILE) $(BUILD_IMAGE_SRCS)
 	$(call build-systemimage-target,$@)