gprs: Create a gprs_utils file and move to be shared code in there

We intend to move some of these routines to libosmocore but to avoid
a feature symbol clash we are prefixing these routines with gprs_.

Done with Jacob
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index 87dbc30..2e26b8a 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -14,7 +14,7 @@
 endif
 
 osmo_gbproxy_SOURCES =  gb_proxy.c gb_proxy_main.c gb_proxy_vty.c \
-			gprs_llc_parse.c crc24.c
+			gprs_llc_parse.c crc24.c gprs_utils.c
 osmo_gbproxy_LDADD = 	$(top_builddir)/src/libcommon/libcommon.a \
 			$(OSMO_LIBS)