ulp: use += instead of := when setting component vars

Component which includes component_ulp_common.mk may also need to set
some of the same COMPONENT_XXX variables. Logically, we should combine
the lists of files to embed, ldflags, extra include dirs, etc.

Fixes https://github.com/espressif/esp-idf/issues/2157.

components/ulp/component_ulp_common.mk

@@ -81,7 +81,7 @@ build: $(COMPONENT_BUILD_DIR)/$(ULP_EXPORTS_HEADER) \
 $(ULP_EXP_DEP_OBJECTS) : $(ULP_EXPORTS_HEADER) $(ULP_SYM)
 
 # Finally, set all the variables processed by the build system. 
-COMPONENT_EXTRA_CLEAN := $(ULP_OBJECTS) \
+COMPONENT_EXTRA_CLEAN += $(ULP_OBJECTS) \
 			$(ULP_LD_SCRIPT) \
 			$(ULP_PREPROCESSED) \
 			$(ULP_ELF) $(ULP_BIN) \
@@ -91,6 +91,6 @@ COMPONENT_EXTRA_CLEAN := $(ULP_OBJECTS) \
 			$(ULP_DEP) \
 			$(ULP_LISTINGS)
 
-COMPONENT_EMBED_FILES := $(COMPONENT_BUILD_DIR)/$(ULP_BIN)
-COMPONENT_ADD_LDFLAGS := -l$(COMPONENT_NAME) -T $(COMPONENT_BUILD_DIR)/$(ULP_EXPORTS_LD)
-COMPONENT_EXTRA_INCLUDES := $(COMPONENT_BUILD_DIR)
+COMPONENT_EMBED_FILES += $(COMPONENT_BUILD_DIR)/$(ULP_BIN)
+COMPONENT_ADD_LDFLAGS += -l$(COMPONENT_NAME) -T $(COMPONENT_BUILD_DIR)/$(ULP_EXPORTS_LD)
+COMPONENT_EXTRA_INCLUDES += $(COMPONENT_BUILD_DIR)