Add support for 32k XTAL as RTC_SLOW_CLK source

- RTC_CNTL_SLOWCLK_FREQ define is removed; rtc_clk_slow_freq_get_hz
  function can be used instead to get an approximate RTC_SLOW_CLK
  frequency

- Clock calibration is performed at startup. The value is saved and used
  for timekeeping and when entering deep sleep.

- When using the 32k XTAL, startup code will wait for the oscillator to
  start up. This can be possibly optimized by starting a separate task
  to wait for oscillator startup, and performing clock switch in that
  task.

- Fix a bug that 32k XTAL would be disabled in rtc_clk_init.

- Fix a rounding error in rtc_clk_cal, which caused systematic frequency
  error.

- Fix an overflow bug which caused rtc_clk_cal to timeout early if the
  slow_clk_cycles argument would exceed certain value

- Improve 32k XTAL oscillator startup time by introducing bootstrapping
  code, which uses internal pullup/pulldown resistors on 32K_N/32K_P
  pins to set better initial conditions for the oscillator.

components/esp32/include/esp_clk.h

@@ -0,0 +1,44 @@
+// Copyright 2015-2017 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.
+
+#pragma once
+
+/**
+ * @file esp_clk.h
+ *
+ * This file contains declarations of clock related functions.
+ * These functions are used in ESP-IDF components, but should not be considered
+ * to be part of public API.
+ */
+
+/**
+ * @brief Initialize clock-related settings
+ *
+ * Called from cpu_start.c, not intended to be called from other places.
+ * This function configures the CPU clock, RTC slow and fast clocks, and
+ * performs RTC slow clock calibration.
+ */
+void esp_clk_init(void);
+
+
+/**
+ * @brief Get the cached calibration value of RTC slow clock
+ *
+ * The value is in the same format as returned by rtc_clk_cal (microseconds,
+ * in Q13.19 fixed-point format).
+ *
+ * @return the calibration value obtained using rtc_clk_cal, at startup time
+ */
+uint32_t esp_clk_slowclk_cal_get();
+