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/tests/gbproxy/gbproxy_test.c b/openbsc/tests/gbproxy/gbproxy_test.c
index 9fceabf..0d14d21 100644
--- a/openbsc/tests/gbproxy/gbproxy_test.c
+++ b/openbsc/tests/gbproxy/gbproxy_test.c
@@ -30,6 +30,7 @@
 #include <osmocom/gprs/gprs_bssgp.h>
 
 #include <openbsc/gb_proxy.h>
+#include <openbsc/gprs_utils.h>
 #include <openbsc/debug.h>
 
 #define REMOTE_BSS_ADDR 0x01020304
@@ -944,7 +945,7 @@
 	gbcfg.core_mcc = 123;
 	gbcfg.core_mnc = 456;
 	gbcfg.core_apn = talloc_zero_size(NULL, 100);
-	gbcfg.core_apn_size = gbprox_str_to_apn(gbcfg.core_apn, "foo.bar", 100);
+	gbcfg.core_apn_size = gprs_str_to_apn(gbcfg.core_apn, "foo.bar", 100);
 
 	configure_sgsn_peer(&sgsn_peer);
 	configure_bss_peers(bss_peer, ARRAY_SIZE(bss_peer));