Add Doxygen configuration file and makefile target

It can be used to generate sometimes useful call-graphs.

.gitignore

@@ -7,3 +7,4 @@ firmware
 .cache
 compile_commands.json
 .vscode
+/docs

Doxyfile

@@ -0,0 +1,17 @@
+OUTPUT_DIRECTORY       = docs
+GENERATE_LATEX         = NO
+GENERATE_RTF           = NO
+GENERATE_MAN           = NO
+OPTIMIZE_OUTPUT_FOR_C  = YES
+HAVE_DOT               = YES
+EXTRACT_ALL            = YES
+EXTRACT_PRIVATE        = YES
+EXTRACT_STATIC         = YES
+CALL_GRAPH             = YES
+CALLER_GRAPH           = YES
+DISABLE_INDEX          = YES
+GENERATE_TREEVIEW      = YES
+RECURSIVE              = YES
+COLLABORATION_GRAPH    = YES
+GRAPHICAL_HIERARCHY    = YES
+DOT_MULTI_TARGETS      = YES

Makefile

@@ -441,3 +441,6 @@ bsp/dp32g030/%.h: hardware/dp32g030/%.def
 
 clean:
 	$(RM) $(call FixPath, $(TARGET).bin $(TARGET).packed.bin $(TARGET) $(OBJS) $(DEPS))
+
+doxygen:
+	doxygen