Use F_CPU if (?) CPU frequency switch is compile-time only (#6833)

* At least the F_CPU define in host mock.h is needed by host Arduino.h - need to include Arduino.h further down in mock.h for this to work.

* Geting the include order right

* Prepare for runtime CPU clock rate selection

* Fix compile for not defined F_CPU

If defined F_CPU, make getCpuFreqMHz() a constexpr

* Use defines for register CPU2X instead of hex value

* Fix build for host - getCpuFreqMHz there was also in conflict with getCycleCount, using F_CPU:

tests/host/common/mock.h:#define F_CPU 80000000 (!)

* Asymmetrical includes and defines on host

* Support restart switch from 160MHz to 80MHz, e.g for OTA. Fixes #579

cores/esp8266/Esp.cpp

@@ -264,11 +264,12 @@ uint8_t EspClass::getBootMode(void)
     return system_get_boot_mode();
 }
 
+#ifndef F_CPU
 uint8_t EspClass::getCpuFreqMHz(void)
 {
     return system_get_cpu_freq();
 }
-
+#endif
 
 uint32_t EspClass::getFlashChipId(void)
 {