v1.8.0: Audio 代码重构与低功耗优化 (#943)

* Reconstruct Audio Code

* Remove old IoT implementation

* Add MQTT-UDP documentation

* OTA升级失败时，可以继续使用

main/audio/codecs/dummy_audio_codec.cc

@@ -0,0 +1,20 @@
+#include "dummy_audio_codec.h"
+
+DummyAudioCodec::DummyAudioCodec(int input_sample_rate, int output_sample_rate) {
+    duplex_ = true;
+    input_reference_ = false;
+    input_channels_ = 1;
+    input_sample_rate_ = input_sample_rate;
+    output_sample_rate_ = output_sample_rate;
+}
+
+DummyAudioCodec::~DummyAudioCodec() {
+}
+
+int DummyAudioCodec::Read(int16_t* dest, int samples) {
+    return 0;
+}
+
+int DummyAudioCodec::Write(const int16_t* data, int samples) {
+    return 0;
+}