tbf: Begin to add some structure to the tbf code

The TBF should use the IMSI to identify a block flow but all
handling is spread across the entire code. Start to clean this
up by moving relevant code into the tbf file. Afterwards one
can clean up and add more internal structure.
diff --git a/src/Makefile.am b/src/Makefile.am
index 38a4b3c..6646f53 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -40,7 +40,8 @@
 	gsm_timer.cpp \
 	bitvector.cpp \
 	pcu_l1_if.cpp \
-	pcu_vty.c
+	pcu_vty.c \
+	tbf.cpp
 
 if ENABLE_SYSMOBTS
 libgprs_la_SOURCES += \
@@ -72,7 +73,8 @@
 	bitvector.h \
 	pcu_vty.h \
 	sysmo_l1_if.h \
-	femtobts.h
+	femtobts.h \
+	tbf.h
 
 osmo_pcu_SOURCES = pcu_main.cpp