Update doxygen annotations in libosmocore

This adds and improves doxygen API descriptions all over libosmocore,
reducing the 'white spots' that don't have any documentation.
diff --git a/src/macaddr.c b/src/macaddr.c
index a6e1304..f83e054 100644
--- a/src/macaddr.c
+++ b/src/macaddr.c
@@ -20,12 +20,24 @@
  *
  */
 
+/*! \addtogroup utils
+ *  @{
+ */
+
+/*! \file loggingrb.c */
+
 #include <stdint.h>
 #include <string.h>
 #include <stdlib.h>
 #include <unistd.h>
 
-
+/*! \brief Parse a MAC address from human-readable notation
+ *  This function parses an ethernet MAC address in the commonly-used
+ *  hex/colon notation (00:00:00:00:00:00) and generates the binary
+ *  representation from it.
+ *  \param[out] out pointer to caller-allocated buffer of 6 bytes
+ *  \param[in] in pointer to input data as string with hex/colon notation
+ */
 int osmo_macaddr_parse(uint8_t *out, const char *in)
 {
 	/* 00:00:00:00:00:00 */
@@ -54,7 +66,11 @@
 #include <net/if_dl.h>
 #include <net/if_types.h>
 
-
+/*! \brief Obtain the MAC address of a given network device
+ *  \param[out] mac_out pointer to caller-allocated buffer of 6 bytes
+ *  \param[in] dev_name string name of the network device
+ *  \returns 0 in case of success; negative otherwise
+ */
 int osmo_get_macaddr(uint8_t *mac_out, const char *dev_name)
 {
 	int rc = -1;
@@ -92,6 +108,11 @@
 #include <netinet/in.h>
 #include <netinet/ip.h>
 
+/*! \brief Obtain the MAC address of a given network device
+ *  \param[out] mac_out pointer to caller-allocated buffer of 6 bytes
+ *  \param[in] dev_name string name of the network device
+ *  \returns 0 in case of success; negative otherwise
+ */
 int osmo_get_macaddr(uint8_t *mac_out, const char *dev_name)
 {
 	int fd, rc;
@@ -114,3 +135,5 @@
 	return 0;
 }
 #endif
+
+/*! @} */