Add new "checksource" target to the library makefiles, for automated source file existance checking.

Demos/Device/ClassDriver/DualVirtualSerial/makefile

@@ -701,6 +701,15 @@ doxygen:
 clean_doxygen:
 	rm -rf Documentation
 
+checksource:
+	@for f in $(SRC) $(CPPSRC) $(ASRC); do \
+		if [ -f $$f ]; then \
+			echo "Found Source File: $$f" ; \
+		else \
+			echo "Source File Not Found: $$f" ; \
+		fi; done 
+
+
 # Create object files directory
 $(shell mkdir $(OBJDIR) 2>/dev/null)
 
@@ -713,5 +722,5 @@ $(shell mkdir $(OBJDIR) 2>/dev/null)
 .PHONY : all begin finish end sizebefore sizeafter gccversion \
 build elf hex eep lss sym coff extcoff doxygen clean          \
 clean_list clean_doxygen program dfu flip flip-ee dfu-ee      \
-debug gdb-config
+debug gdb-config checksource