introduce byteswap.h with osmo_{htonl,ntohl,htons,ntohs}

We need to have an architecture-independend way of endian conversion /
byte swapping functions which will also work on embedded (bare iron)
builds.   Let's introduce osmocom/core/bytesawp.h for this purpose.

Change-Id: Ibc0cc1e36d4ed63a35cf8ceff3af0f26e5ac7a3d
diff --git a/tests/Makefile.am b/tests/Makefile.am
index d9816ef..d95083b 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -16,7 +16,7 @@
 		 tlv/tlv_test gsup/gsup_test oap/oap_test fsm/fsm_test	\
 		 write_queue/wqueue_test socket/socket_test		\
 		 coding/coding_test conv/conv_gsm0503_test		\
-		 abis/abis_test
+		 abis/abis_test endian/endian_test
 
 if ENABLE_MSGFILE
 check_PROGRAMS += msgfile/msgfile_test
@@ -162,6 +162,9 @@
   $(top_builddir)/src/codec/libosmocodec.la \
   $(top_builddir)/src/coding/libosmocoding.la
 
+endian_endian_test_SOURCES = endian/endian_test.c
+endian_endian_test_LDADD = $(top_builddir)/src/libosmocore.la
+
 # The `:;' works around a Bash 3.2 bug when the output is not writeable.
 $(srcdir)/package.m4: $(top_srcdir)/configure.ac
 	:;{ \
@@ -204,7 +207,7 @@
 	     osmo-auc-gen/osmo-auc-gen_test.sh				\
 	     osmo-auc-gen/osmo-auc-gen_test.ok				\
 	     osmo-auc-gen/osmo-auc-gen_test.err				\
-	     conv/conv_gsm0503_test.ok
+	     conv/conv_gsm0503_test.ok endian/endian_test.ok
 
 DISTCLEANFILES = atconfig atlocal conv/gsm0503_test_vectors.c
 BUILT_SOURCES = conv/gsm0503_test_vectors.c