esp_hw_support: esp_clk should use spinlock instead of mutex

esp_clk used to be protected using _lock_t (i.e., a FreeRTOS Mutex). However, esp_clk API is current called from
from critical sections, thus mutex should not be used (as they can be blocking).

This commit updates esp_clk to use spinlocks for critical sections instead.

Note: Added a small fix to exclude esp_clk.c from bootloader builds in the
      legacy build system (this is already the case in CMake).

components/esp_hw_support/component.mk

@@ -4,6 +4,7 @@ COMPONENT_ADD_LDFRAGMENTS := linker.lf
 
 ifdef IS_BOOTLOADER_BUILD
 COMPONENT_OBJEXCLUDE += clk_ctrl_os.o \
+						esp_clk.o \
 						intr_alloc.o \
 						sleep_modes.o \
 						sleep_gpio.o \

components/esp_hw_support/esp_clk.c

@@ -8,6 +8,7 @@
 #include <sys/param.h>
 #include <sys/lock.h>
 
+#include "freertos/FreeRTOS.h"
 #include "esp_attr.h"
 #include "soc/rtc.h"
 
@@ -44,7 +45,7 @@ extern uint32_t g_ticks_per_us_app;
 #endif
 #endif
 
-static _lock_t s_esp_rtc_time_lock;
+static portMUX_TYPE s_esp_rtc_time_lock = portMUX_INITIALIZER_UNLOCKED;
 static RTC_DATA_ATTR uint64_t s_esp_rtc_time_us = 0, s_rtc_last_ticks = 0;
 
 inline static int IRAM_ATTR s_get_cpu_freq_mhz(void)
@@ -86,7 +87,7 @@ void IRAM_ATTR ets_update_cpu_frequency(uint32_t ticks_per_us)
 
 uint64_t esp_rtc_get_time_us(void)
 {
-    _lock_acquire(&s_esp_rtc_time_lock);
+    portENTER_CRITICAL_SAFE(&s_esp_rtc_time_lock);
     const uint32_t cal = esp_clk_slowclk_cal_get();
     const uint64_t rtc_this_ticks = rtc_time_get();
     const uint64_t ticks = rtc_this_ticks - s_rtc_last_ticks;
@@ -107,7 +108,7 @@ uint64_t esp_rtc_get_time_us(void)
            ((ticks_high * cal) << (32 - RTC_CLK_CAL_FRACT));
     s_esp_rtc_time_us += delta_time_us;
     s_rtc_last_ticks = rtc_this_ticks;
-    _lock_release(&s_esp_rtc_time_lock);
+    portEXIT_CRITICAL_SAFE(&s_esp_rtc_time_lock);
     return s_esp_rtc_time_us;
 }