build: support multiple source files and nested directories for userspace tools

tools/tool-template.mk

@@ -1,13 +1,13 @@
 BUILD_DIR := $(TOOLS_BUILD_DIR)/$(TOOL_NAME)
 
-SRC := $(wildcard *.c)
+SRC := $(shell find . -name '*.c')
 OBJ := $(addprefix $(BUILD_DIR)/,$(SRC:.c=.o))
 
 $(BUILD_DIR)/$(TOOL_NAME): $(OBJ)
 	@mkdir -p $(@D)
 
 	@printf "    \033[1;36mHOSTLD\033[0m \033[1mtools/$(TOOL_NAME)\033[0m\n"
-	@$(CC) $< -o $@ $(CFLAGS) $(LDFLAGS)
+	@$(CC) $^ -o $@ $(CFLAGS) $(LDFLAGS)
 
 $(BUILD_DIR)/%.o: %.c
 	@printf "    \033[1;32mHOSTCC\033[0m $(TOOL_NAME)/$^\n"