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/endian/endian_test.ok b/tests/endian/endian_test.ok
new file mode 100644
index 0000000..a42f457
--- /dev/null
+++ b/tests/endian/endian_test.ok
@@ -0,0 +1,3 @@
+Testing 16bit swappinng
+Testing 32bit swappinng
+Testing ntohX() and htonX()