Allow to easily build the webserver example with cfs support by running make with the parameter HTTPD-CFS=1.

examples/webserver/Makefile

@@ -1,5 +1,37 @@
 all: webserver-example
 APPS=webserver
 
+# The webserver application normally contains a built-in file system and support
+# for server-side includes.
+#
+# This webserver example supports building the alternative webserver application
+# which serves files from an cfs file system. To build the alternative webserver
+# run make with the parameter HTTPD-CFS=1.
+
+ifeq ($(HTTPD-CFS),1)
+override webserver_src = webserver-nogui.c http-strings.c psock.c memb.c \
+                         httpd-cfs.c
+endif
+
 CONTIKI = ../..
 include $(CONTIKI)/Makefile.include
+
+# Intentionally httpd.c and httpd-cfs.c implement the same interface. When
+# switching from one webserver alternative to the other with an existent
+# Contiki library then both files end up in the library making the linker
+# use whichever it finds first :-(
+#
+# The most straightforward way to make sure this doesn't happen is to delete
+# the Contiki library. But it would be undesirable to do that on every build
+# so the existence of the "wrong" object file is used to detect a switch and
+# trigger deletion of the Contiki library - and the trigger file of course.
+
+ifeq ($(HTTPD-CFS),1)
+  ifneq (${wildcard $(OBJECTDIR)/httpd.o},)
+  DUMMY := ${shell rm -f contiki-$(TARGET).a $(OBJECTDIR)/httpd.o}
+  endif
+else
+  ifneq (${wildcard $(OBJECTDIR)/httpd-cfs.o},)
+  DUMMY := ${shell rm -f contiki-$(TARGET).a $(OBJECTDIR)/httpd-cfs.o}
+  endif
+endif