gprs: Separate LLC parsing from LLC state handling

Currently LLC parsing is part of gprs_llc.c which needs large parts
of the SGSN code parsing to fulfill its link dependencies.

This patch moves the functions that just do plain parsing, dumping,
and FCS computation to a different file to avoid these dependencies
if LLC stateful processing is not needed. It also exposes
struct gprs_llc_hdr_parsed and enum gprs_llc_cmd publically.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index 90c376c..049d41d 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -19,7 +19,7 @@
 
 osmo_sgsn_SOURCES =	gprs_gmm.c gprs_sgsn.c gprs_sndcp.c gprs_sndcp_vty.c \
 			sgsn_main.c sgsn_vty.c sgsn_libgtp.c \
-			gprs_llc.c gprs_llc_vty.c crc24.c \
+			gprs_llc.c gprs_llc_parse.c gprs_llc_vty.c crc24.c \
 			sgsn_ctrl.c
 osmo_sgsn_LDADD = 	\
 			$(top_builddir)/src/libctrl/libctrl.a \