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/src/libcommon/Makefile.am b/openbsc/src/libcommon/Makefile.am
index 75f40ee..84c7544 100644
--- a/openbsc/src/libcommon/Makefile.am
+++ b/openbsc/src/libcommon/Makefile.am
@@ -6,4 +6,4 @@
 
 libcommon_a_SOURCES = bsc_version.c common_vty.c debug.c gsm_data.c \
 		      gsm_data_shared.c socket.c talloc_ctx.c \
-		      gsm_subscriber_base.c
+		      gsm_subscriber_base.c utils.c