Enable dynamic log level selection (#1492)

This commit adds support for the dynamic log level selection. Config
option `OPENTHREAD_CONFIG_ENABLE_DYNAMIC_LOG_LEVEL` can be used to
enable/disable this feature (default setting is disabled).

Note that the `OPENTHREAD_CONFIG_LOG_LEVEL` specifies the log level at
compile/build time. The dynamic log level feature (if enabled) only
allows decreasing log level from the compile time setting.

This commit also adds `SPINEL_PROP_DEBUG_NCP_LOG_LEVEL` as a new
spinel property to control the log level. The set/get handlers for
this property along with its documentation are also added.

This commit modifies some of the log related macros/methods so
to harmonize them all to require an `otInstance` as their first
argument.

src/core/api/instance_api.cpp

@@ -33,6 +33,12 @@
 
 #define WPP_NAME "instance_api.tmh"
 
+#ifdef OPENTHREAD_CONFIG_FILE
+#include OPENTHREAD_CONFIG_FILE
+#else
+#include <openthread-config.h>
+#endif
+
 #include "openthread/instance.h"
 #include "openthread/platform/misc.h"
 #include "openthread/platform/settings.h"
@@ -61,6 +67,9 @@ otInstance::otInstance(void) :
 #if OPENTHREAD_ENABLE_APPLICATION_COAP
     , mApplicationCoapServer(mIp6.mUdp, OT_DEFAULT_COAP_PORT)
 #endif // OPENTHREAD_ENABLE_APPLICATION_COAP
+#if OPENTHREAD_CONFIG_ENABLE_DYNAMIC_LOG_LEVEL
+    , mLogLevel(static_cast<otLogLevel>(OPENTHREAD_CONFIG_LOG_LEVEL))
+#endif // OPENTHREAD_CONFIG_ENABLE_DYNAMIC_LOG_LEVEL
 {
 }
 
@@ -214,6 +223,35 @@ exit:
     return error;
 }
 
+otLogLevel otGetDynamicLogLevel(otInstance *aInstance)
+{
+    otLogLevel logLevel;
+
+#if OPENTHREAD_CONFIG_ENABLE_DYNAMIC_LOG_LEVEL
+    logLevel =  aInstance->mLogLevel;
+#else
+    logLevel = static_cast<otLogLevel>(OPENTHREAD_CONFIG_LOG_LEVEL);
+    (void)aInstance;
+#endif
+
+    return logLevel;
+}
+
+ThreadError otSetDynamicLogLevel(otInstance *aInstance, otLogLevel aLogLevel)
+{
+    ThreadError error = kThreadError_None;
+
+#if OPENTHREAD_CONFIG_ENABLE_DYNAMIC_LOG_LEVEL
+    aInstance->mLogLevel = aLogLevel;
+#else
+    error = kThreadError_NotCapable;
+    (void)aInstance;
+    (void)aLogLevel;
+#endif
+
+    return error;
+}
+
 #ifdef __cplusplus
 }  // extern "C"
 #endif