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/Doxyfile

@@ -155,6 +155,8 @@ INPUT = \
     ../../components/esp32/include/esp32/pm.h \
     ### esp_timer, High Resolution Timer
     ../../components/esp32/include/esp_timer.h \
+    ### ESP Pthread parameters
+    ../../components/pthread/include/esp_pthread.h \
     ###
     ### FreeRTOS
     ###