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/cpu_start.c

@@ -1,9 +1,9 @@
-// Copyright 2015-2016 Espressif Systems (Shanghai) PTE LTD
+// 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
@@ -11,6 +11,7 @@
 // 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.
+
 #include <stdint.h>
 #include <string.h>
 
@@ -60,6 +61,7 @@
 #include "esp_panic.h"
 #include "esp_core_dump.h"
 #include "esp_app_trace.h"
+#include "esp_clk.h"
 #include "trax.h"
 
 #define STRINGIFY(s) STRINGIFY2(s)
@@ -202,7 +204,7 @@ void start_cpu0_default(void)
 #endif
     trax_start_trace(TRAX_DOWNCOUNT_WORDS);
 #endif
-    esp_set_cpu_freq();     // set CPU frequency configured in menuconfig
+    esp_clk_init();
 #ifndef CONFIG_CONSOLE_UART_NONE
     uart_div_modify(CONFIG_CONSOLE_UART_NUM, (rtc_clk_apb_freq_get() << 4) / CONFIG_CONSOLE_UART_BAUDRATE);
 #endif