Honour values of configCPU_CLOCK_HZ & configTICK_RATE_HZ for tick rate

Fixes #147

* Can vary tick rate from 100Hz via configTICK_RATE_HZ. Note that the
  SDK binary libraries are hard-coded to assume the tick rate is 100Hz,
  so changing the tick rate may have unexpected consequences for lower
  layer WiFi behaviour (such as certain kinds of timeouts happening
  faster/slower.)

* Setting configCPU_CLOCK_HZ to 160MHz means ESP will set 160MHz during
  initialisation. Only 80MHz and 160MHz are supported.

* Timing of tasks is no longer affected by current CPU freq (whether set
  via configCPU_CLOCK_HZ or via sdk_system_update_cpu_freq().)
  Previously doubling the CPU frequency would double the tick rate.

open_esplibs/include/esplibs/libmain.h

@@ -1,3 +1,4 @@
+#include "sdk_internal.h"
 #ifndef _ESPLIBS_LIBMAIN_H
 #define _ESPLIBS_LIBMAIN_H
 
@@ -5,6 +6,9 @@
 
 // misc.c
 int sdk_os_get_cpu_frequency(void);
+
+/* Don't call this function from user code, it doesn't change the CPU
+ * speed. Call sdk_system_update_cpu_freq() instead. */
 void sdk_os_update_cpu_frequency(int freq);
 
 // user_interface.c