sba: Move the sba code to a dedicated file

Move the code around and then turn it into an object...
diff --git a/src/Makefile.am b/src/Makefile.am
index cbb6de0..813dfbe 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -46,7 +46,8 @@
 	bts.cpp \
 	poll_controller.cpp \
 	encoding.cpp \
-	ta.cpp
+	ta.cpp \
+	sba.cpp
 
 if ENABLE_SYSMOBTS
 libgprs_la_SOURCES += \