Move NULL and ASSIGN tbf_state transition to tbf_fsm

At some point later in time the state_flags will most probably be split
into different variables, one ending up in a different FSM. It is moved
so far to the exsiting FSM from the C++ class since it's easier to
access it from C and C++ code, and anyway that kind of information
belongs to the FSM.

Related: OS#2709
Change-Id: I3c62e9e83965cb28065338733f182863e54d7474
diff --git a/src/tbf_fsm.h b/src/tbf_fsm.h
index 84bb437..ed025ca 100644
--- a/src/tbf_fsm.h
+++ b/src/tbf_fsm.h
@@ -27,7 +27,9 @@
 struct gprs_rlcmac_tbf;
 
 enum tbf_fsm_event {
-	TBF_EV_FOOBAR,
+	TBF_EV_ASSIGN_ADD_CCCH, /* An assignment is sent over CCCH and confirmation from MS is pending */
+	TBF_EV_ASSIGN_ADD_PACCH, /* An assignment is sent over PACCH and confirmation from MS is pending */
+	TBF_EV_ASSIGN_DEL_CCCH, /* An assignment previously sent over CCCH has been confirmed by MS */
 };
 
 enum tbf_fsm_states {
@@ -42,6 +44,7 @@
 struct tbf_fsm_ctx {
 	struct osmo_fsm_inst *fi;
 	struct gprs_rlcmac_tbf* tbf; /* back pointer */
+	uint32_t state_flags;
 };
 
 extern const struct osmo_tdef_state_timeout tbf_fsm_timeouts[32];