Consolidate build directory creation

Add BUILD_DIRS_ALL to aggregate $(BUILD_DIRS), $(EXTRA_C_BUILD_DIRS), $(EXTRA_CPP_BUILD_DIRS) and lib_gd32, and simplify the builddirs rule to run a single mkdir -p on this variable. Removes the conditional mkdir checks and the explicit lib_gd32 mkdir, reducing duplication and simplifying the Makefile without changing behavior.

Author: vanvught

firmware-template-gd32/lib/Rules.mk

@@ -77,11 +77,10 @@ all : builddirs $(TARGET)
 
 .PHONY: clean builddirs
 
+BUILD_DIRS_ALL := $(BUILD_DIRS) $(EXTRA_C_BUILD_DIRS) $(EXTRA_CPP_BUILD_DIRS) lib_gd32
+
 builddirs:
-	mkdir -p $(BUILD_DIRS) 
-	if [[ -n "${EXTRA_C_BUILD_DIRS}" ]]; then mkdir -p $(EXTRA_C_BUILD_DIRS); fi
-	if [[ -n "${EXTRA_CPP_BUILD_DIRS}" ]]; then mkdir -p $(EXTRA_CPP_BUILD_DIRS); fi
-	mkdir -p lib_gd32
+	mkdir -p $(BUILD_DIRS_ALL)
 
 clean:
 	rm -rf build_gd32