brownout detector: enable by default

Because of errata related to BOD reset function, brownout is handled as follows:

- attach an ISR to brownout interrupt
- when ISR happens, print a message and do a software restart
- esp_restart_nonos enables RTC watchdog, so if restart fails,
  there will be  one more attempt to restart (using the RTC
  watchdog)

components/esp32/include/esp_brownout.h

@@ -0,0 +1,21 @@
+// Copyright 2015-2016 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+
+#ifndef __ESP_BROWNOUT_H
+#define __ESP_BROWNOUT_H
+
+void esp_brownout_init();
+
+#endif

components/esp32/include/esp_system.h

@@ -59,6 +59,17 @@ void system_restore(void) __attribute__ ((deprecated));
   */
 void esp_restart(void) __attribute__ ((noreturn));
 
+/**
+  * @brief  Internal function to restart PRO and APP CPUs.
+  *
+  * @note This function should not be called from FreeRTOS applications.
+  *       Use esp_restart instead.
+  *
+  * This is an internal function called by esp_restart. It is called directly
+  * by the panic handler and brownout detector interrupt.
+  */
+void esp_restart_noos() __attribute__ ((noreturn));
+
 /**
   * @brief  Restart system.
   *