core/msgb.h: introduce msgb_l4len() helper
There is already a group of similar functions for L1, L2 and L3,
but L4 was missing. The L4 is usually used for parsing of complex
L3 messages, such as SS/USSD and SMS.
This change introduces a similar halper for L4.
Change-Id: I755f2d654bbdad2a8b4f94df9023bdd370b07ae6
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 64833d2..5222ec9 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -10,3 +10,4 @@
libosmogsm gsm0480_l3hdr_push() removed from gsm/gsm0480.h (was not exposed)
libosmogsm gsm48_push_l3hdr() (re)introduced in gsm/gsm48.h (GSM 04.08 API)
libosmogsm gsm48_push_l3hdr_tid() a wrapper around gsm48_push_l3hdr()
+libosmocore msgb_l4len new symbol (exposed as 'static inline' in msgb.h)