nanocoap: Add coap_opt_add_opaque

This option complements the existing coap_opt_add_{uint,string} and even
more special-purpose functions; its implementation is trivial given the
existing static _add_opt_pkt function.

The method is useful when working with ETags (ETag, If-Match options).

sys/include/net/nanocoap.h

@@ -656,6 +656,23 @@ size_t coap_put_block1_ok(uint8_t *pkt_pos, coap_block1_t *block1, uint16_t last
  */
 ssize_t coap_opt_add_string(coap_pkt_t *pkt, uint16_t optnum, const char *string, char separator);
 
+/**
+ * @brief   Encode the given buffer as an opaque data option into pkt
+ *
+ * @post pkt.payload advanced to first byte after option(s)
+ * @post pkt.payload_len reduced by option(s) length
+ *
+ * @param[in,out] pkt         pkt referencing target buffer
+ * @param[in]     optnum      option number to use
+ * @param[in]     val         pointer to the value to be set
+ * @param[in]     val_len     length of val
+ *
+ * @return        number of bytes written to buffer
+ * @return        <0 on error
+ * @return        -ENOSPC if no available options
+ */
+ssize_t coap_opt_add_opaque(coap_pkt_t *pkt, uint16_t optnum, const uint8_t *val, size_t val_len);
+
 /**
  * @brief   Encode the given uint option into pkt
  *

sys/net/application_layer/nanocoap/nanocoap.c

@@ -802,6 +802,11 @@ ssize_t coap_opt_add_string(coap_pkt_t *pkt, uint16_t optnum, const char *string
     return write_len;
 }
 
+ssize_t coap_opt_add_opaque(coap_pkt_t *pkt, uint16_t optnum, uint8_t *val, size_t val_len)
+{
+    return _add_opt_pkt(pkt, optnum, val, val_len);
+}
+
 ssize_t coap_opt_add_uint(coap_pkt_t *pkt, uint16_t optnum, uint32_t value)
 {
     uint32_t tmp = value;