components/esp32: add CPU frequency selection in menuconfig

Note that with WiFi stack enabled, system_init will reset frequency to 240MHz.
To make this setting useful, esp32-wifi-libs submodule needs to be updated.

components/esp32/include/soc/cpu.h

@@ -33,4 +33,14 @@ static inline bool cpu_in_interrupt_context(void)
     return (ps & PS_UM) == 0;
 }
 
+
+/*
+ * @brief Set CPU frequency to the value defined in menuconfig
+ *
+ * Called from cpu_start.c, not intended to be called from other places.
+ * This is a temporary function which will be replaced once dynamic
+ * CPU frequency changing is implemented.
+ */
+void esp_set_cpu_freq();
+
 #endif