Fix source path and include flag for Contiki build

As the libcoap sources have been moved to the sub-directory src it
must be added to the vpath. The header files now reside in include/coap,
so we need an additional -I compiler flag for this.

The root path for libcoap is derived from the variable APPDS which is
set by $(CONTIKI)/Makefile.include. This is a bit fragile but the best
we have...

Makefile.libcoap

@@ -1 +1,7 @@
 libcoap_src = pdu.c net.c debug.c encode.c uri.c subscribe.c resource.c hashkey.c str.c option.c async.c block.c mem.c coap_io.c
+
+libcoap_dir := $(filter %libcoap,$(APPDS))
+vpath %c $(libcoap_dir)/src
+
+# set include path for coap sources
+CFLAGS += -I$(libcoap_dir)/include/coap