Fix FreeRTOS.h/semphr.h include paths for ESP-IDF (fixes #1439) (#1570)

mutex_freertos.h unconditionally included the vanilla FreeRTOS header
layout (FreeRTOS.h and semphr.h at the include root). On ESP-IDF, and
therefore Arduino-ESP32 as well since it's built on top of ESP-IDF,
these headers are nested one level deeper under freertos/, so the
unconditional include fails to compile on any ESP32 target with
ETL_TARGET_OS_FREERTOS defined.

Use __has_include to try the standard path first, fall back to the
ESP-IDF nested path, and fail with a clear #error if neither is found,
rather than a mysterious "file not found" from the compiler.

Also resolves a pre-existing include-style inconsistency in this file:
the old code mixed "FreeRTOS.h" (quotes) with <semphr.h> (angle
brackets). All includes now uniformly use angle brackets.

include/etl/mutex/mutex_freertos.h

@@ -31,8 +31,26 @@ SOFTWARE.
 
 #include "../platform.h"
 
-#include "FreeRTOS.h"
+#if defined(__has_include) // Compiler supports __has_include
-#include <semphr.h>
+
+  #if __has_include(<FreeRTOS.h>) && __has_include(<semphr.h>) // Standard FreeRTOS include layout
+    #include <FreeRTOS.h>
+    #include <semphr.h>
+
+  #elif __has_include(<freertos/FreeRTOS.h>) && __has_include(<freertos/semphr.h>) // ESP-IDF style include layout
+    #include <freertos/FreeRTOS.h>
+    #include <freertos/semphr.h>
+
+  #else // Neither include layout found
+    #error "FreeRTOS.h and semphr.h not found. Please include path to FreeRTOS in your project settings"
+  #endif
+
+#else
+
+  #include <FreeRTOS.h>
+  #include <semphr.h>
+
+#endif
 
 namespace etl
 {