libcommon: soak up three static functions.

Add new kitchen sink openbsc/utils.h and libcommon/utils.c to make three so far
static functions public (so I can use them in the upcoming OAP code).

A place to put them could have been the gprs_utils.h, but all general functions
in there have a gprs_ prefix, and todo markings to move them away. All other
libcommon headers are too specific, so I opened up this kitchen sink header.

Replace the implementation of encode_big_endian() with a call to
osmo_store64be_ext(). See comments.

Apply the change in Makefiles and C files.
diff --git a/openbsc/tests/gprs/Makefile.am b/openbsc/tests/gprs/Makefile.am
index 633c362..b57977b 100644
--- a/openbsc/tests/gprs/Makefile.am
+++ b/openbsc/tests/gprs/Makefile.am
@@ -6,6 +6,7 @@
 noinst_PROGRAMS = gprs_test
 
 gprs_test_SOURCES = gprs_test.c $(top_srcdir)/src/gprs/gprs_utils.c \
-		$(top_srcdir)/src/gprs/gprs_gsup_messages.c
+		$(top_srcdir)/src/gprs/gprs_gsup_messages.c \
+		$(top_srcdir)/src/libcommon/utils.c
 
 gprs_test_LDADD = $(LIBOSMOCORE_LIBS) $(LIBOSMOGSM_LIBS)