Feature/makefile add apps library (#218)

* add BACnet stack library at apps/lib

* convert apps to use apps/lib for smaller binary

* fix -DBACDL_ALL=1 build

* fix piface build

* datalink MAX_MPDU and MAX_HEADER cleanup

* add bip6 to git workflow

* fix system library dependency of BACnet library

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

apps/piface/configure.sh

@@ -3,8 +3,15 @@
 # The library is located at github.com/piface
 # Since the PiFace library is GPLv3, we have to keep it separate.
 
-git clone https://github.com/piface/libmcp23s17.git
-git clone https://github.com/piface/libpifacedigital.git
+if [ ! -d "libmcp23s17" ]
+then
+    git clone git@github.com:piface/libmcp23s17.git
+fi
+
+if [ ! -d "libpifacedigital" ]
+then
+    git clone git@github.com:piface/libpifacedigital.git
+fi
 
 # Build the library