Auto-initialize optional submodules when needed

Makefile

@@ -22,6 +22,7 @@ OBJCOPY   := avr-objcopy
 AVD       := avrdude
 MKDIR     := mkdir -p
 RMR       := rm -rf
+GIT       := git
 
 SRCDIR    := src
 BINDIR    := bin
@@ -79,19 +80,24 @@ distclean: clean
 	$(Q) $(RMR) $(BINDIR)
 
 .PHONY: listen
-listen: ./opt/tools/serial-listen.py
-	$(E) "[PY3] $<"
+listen: opt/tools/serial-listen
+	$(E) "[RUN] $<"
 	$(Q) ./$<
 
 .PHONY: webgui
-webgui: ./opt/webgui/Makefile
-	$(E) "[MAK] $(<D)"
+webgui: opt/webgui/Makefile
+	$(E) "[MAK] $<"
 	$(Q) $(MAKE) -sC $(<D)
 
 $(TMPDIRS):
 	$(E) "[DIR] $@"
 	$(Q) $(MKDIR) $@
 
+# Initialize optional git submodules if needed
+opt/tools/serial-listen opt/webgui/Makefile:
+	$(E) "[GIT] Initializing submodules..."
+	$(Q) $(GIT) submodule update --init
+
 # ==============================================================================
 # PRIMARY BUILD TARGET AND PATTERN RULES
 # ==============================================================================