Add check for unresolved-but-unused symbols at build.

From what I can tell they *should* be harmless, but I'd rather we keep a
neat house in the first place.

Makefile

@@ -188,6 +188,7 @@ endef
 
 $(BINODIR)/%.bin: $(IMAGEODIR)/%.out
 	@mkdir -p $(BINODIR)
+	@$(NM) $< | grep -w U && { echo "Firmware has unresolved (but unused) symbols - should be fixed nevertheless!"; exit 1; } || true
 	$(ESPTOOL) elf2image --flash_mode dio --flash_freq 40m $< -o $(FIRMWAREDIR)
 
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