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/include/Makefile.am b/include/Makefile.am
index 0383d7a..6981c37 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -10,6 +10,7 @@
                        osmocom/core/bits.h \
                        osmocom/core/bitvec.h \
                        osmocom/core/bitcomp.h \
+                       osmocom/core/byteswap.h \
                        osmocom/core/conv.h \
                        osmocom/core/crc16.h \
                        osmocom/core/crc16gen.h \