Added tetra rx ext app (#3257)

firmware/application/external/tetra_rx/tetra_crc.hpp

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <stdint.h>
+
+namespace ui::external_app::tetra_rx {
+
+class CRC {
+   public:
+    static constexpr uint16_t CRC_OK = 0x1d0f;
+
+    static uint16_t crc16_itut_bits(
+        const uint8_t* bits,
+        uint32_t n_bits,
+        uint16_t init = 0xffff);
+};
+
+}  // namespace ui::external_app::tetra_rx