Feature/apdu null length returned (#285)

* Add APDU as NULL to get BACnet type lengths.

* Fix bacapp copy test to succeed

* fix BACnet REAL and DOUBLE decode

* Add unit test for NULL APDU encoding for length

* Add unit tests for bacapp context

* refactor host-n-port to hostnport.c module

* fix BVLC decoder

* additional unit testing for bacapp

* include bacdevobjpropref module in builds

* simplify bacapp snprintf to be able to return length

* adjust compiler for variable-length arrays

* fix bug found by scan-build

Authored-by: Steve Karg <skarg@users.sourceforge.net>

ports/stm32f4xx/Makefile

@@ -53,6 +53,7 @@ BACNET_SRC = \
 	$(BACNET_CORE)/bacaddr.c \
 	$(BACNET_CORE)/bacapp.c \
 	$(BACNET_CORE)/bacdcode.c \
+	$(BACNET_CORE)/bacdevobjpropref.c \
 	$(BACNET_CORE)/bacerror.c \
 	$(BACNET_CORE)/bacint.c \
 	$(BACNET_CORE)/bacreal.c \
@@ -61,6 +62,7 @@ BACNET_SRC = \
 	$(BACNET_CORE)/dcc.c \
 	$(BACNET_CORE)/iam.c \
 	$(BACNET_CORE)/ihave.c \
+	$(BACNET_CORE)/hostnport.c \
 	$(BACNET_CORE)/lighting.c \
 	$(BACNET_CORE)/memcopy.c \
 	$(BACNET_CORE)/npdu.c \