pthread: Allow configuration of priority and stacksize

The expected usage is:
  esp_pthread_set_cfg(cfg);
  pthread_create()

If the inherit flag is set, then all subsequent threads forked by this
thread will also inherit this configuration. This avoids having to
change/prefix this for each and every pthread_create() call.

docs/en/api-reference/system/esp_pthread.rst

@@ -0,0 +1,65 @@
+ESP-pthread
+===========
+
+Overview
+--------
+
+This module offers Espressif specific extensions to the pthread library that can be used to influence the behaviour of pthreads. Currently the following configuration can be tuned:
+  * Stack size of the pthreads
+  * Priority of the created pthreads
+  * Inheriting this configuration across threads
+
+Example to tune the stack size of the pthread:
+
+.. highlight:: c
+
+::
+   
+   main()
+   {
+       pthread_t t1;
+
+       esp_pthread_cfg_t cfg;
+       cfg.stack_size = (4 * 1024);
+       esp_pthread_set_cfg(&cfg);
+
+       pthread_create(&t1, NULL, thread_func);
+   }
+
+The API can also be used for inheriting the settings across threads. For example:
+
+.. highlight:: c
+
+::
+   
+   void * my_thread2(void * p)
+   {
+         /* This thread will inherit the stack size of 4K */
+         printf("In my_thread2\n");
+   }
+
+   void * my_thread1(void * p)
+   {
+         printf("In my_thread1\n");
+	 pthread_t t2;
+	 pthread_create(&t2, NULL, my_thread2);
+   }
+
+   main()
+   {
+
+        pthread_t t1;
+
+        esp_pthread_cfg_t cfg;
+        cfg.stack_size = (4 * 1024);
+        cfg.inherit_cfg = true;
+        esp_pthread_set_cfg(&cfg);
+
+        pthread_create(&t1, NULL, my_thread1);
+   }
+
+API Reference
+-------------
+
+.. include:: /_build/inc/esp_pthread.inc
+

docs/en/api-reference/system/index.rst

@@ -18,6 +18,7 @@ System API
    Sleep Modes <sleep_modes>
    Base MAC address <base_mac_address>
    Over The Air Updates (OTA) <ota>
+   ESP pthread <esp_pthread>
 
 
 Example code for this API section is provided in :example:`system` directory of ESP-IDF examples.