Move NUM_BYTES macro to core library

It's universally useful so it make sense to have it in the shared core:
* move macro from libosmocoding to libosmocore
* add OSMO_ prefix
* add doxygen docs

Change-Id: I5386ba3e1f1cc153ba96c29dc71c9075a052aa02
diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c
index 7039863..1e70d2e 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -901,7 +901,7 @@
 		u, 0, data_len, 1);
 
 	/* Return the number of bytes required for the bit message */
-	return NUM_BYTES(code->hdr_len + code->data_len);
+	return OSMO_BYTES_FOR_BITS(code->hdr_len + code->data_len);
 }
 
 /*! \brief Decode EGPRS UL message