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/testsuite.at b/tests/testsuite.at
index 2f9cfe7..4c8950c 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -268,3 +268,9 @@
 cat $abs_srcdir/osmo-auc-gen/osmo-auc-gen_test.err > experr
 AT_CHECK([$abs_top_srcdir/tests/osmo-auc-gen/osmo-auc-gen_test.sh $abs_top_builddir/utils/osmo-auc-gen], [0], [expout], [experr])
 AT_CLEANUP
+
+AT_SETUP([endian])
+AT_KEYWORDS([endian])
+cat $abs_srcdir/endian/endian_test.ok > expout
+AT_CHECK([$abs_top_builddir/tests/endian/endian_test], [0], [expout], [ignore])
+AT_CLEANUP