nimble/mesh: Add support for log modules

This PR adds support for various log modules which is possible
thanks to modlog feature. With this feature we can specify
a logging level for each module separately which could have the
following advantages:

- Being able to filter out only the logs that we are interested in at
  the moment
- Controlling log level in runtime
- Improved readability if we start printing module name in log header
- Ability to use color-coded log messages by mapping module to a
  specific color

apps/blemesh_shell/src/main.c

@@ -17,6 +17,8 @@
  * under the License.
  */
 
+#define MESH_LOG_MODULE BLE_MESH_LOG
+
 #include <assert.h>
 #include "os/mynewt.h"
 #include "mesh/mesh.h"

apps/blemesh_shell/syscfg.yml

@@ -53,16 +53,3 @@ syscfg.vals:
     BLE_MESH_CFG_CLI: 1
     BLE_MESH_SETTINGS: 0
     CONFIG_NFFS: 0
-
-    BLE_MESH_DEBUG: 0
-    BLE_MESH_DEBUG_NET: 0
-    BLE_MESH_DEBUG_TRANS: 0
-    BLE_MESH_DEBUG_BEACON: 0
-    BLE_MESH_DEBUG_CRYPTO: 0
-    BLE_MESH_DEBUG_PROV: 0
-    BLE_MESH_DEBUG_ACCESS: 0
-    BLE_MESH_DEBUG_MODEL: 0
-    BLE_MESH_DEBUG_ADV: 0
-    BLE_MESH_DEBUG_LOW_POWER: 0
-    BLE_MESH_DEBUG_FRIEND: 0
-    BLE_MESH_DEBUG_PROXY: 0