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/Makefile.am b/openbsc/tests/gbproxy/Makefile.am
index 9c1b2d0..409d791 100644
--- a/openbsc/tests/gbproxy/Makefile.am
+++ b/openbsc/tests/gbproxy/Makefile.am
@@ -11,6 +11,7 @@
 			$(top_builddir)/src/gprs/gb_proxy.o \
 			$(top_builddir)/src/gprs/gprs_llc_parse.o \
 			$(top_builddir)/src/gprs/crc24.o \
+			$(top_builddir)/src/gprs/gprs_utils.o \
 			$(top_builddir)/src/libcommon/libcommon.a \
 			$(top_builddir)/src/libbsc/libbsc.a \
 			$(top_builddir)/src/libtrau/libtrau.a \