Add source code for functions touching the struct netif to better support lwip development: sdk_wifi_get_ip_info, sdk_wifi_set_ip_info, sdk_wifi_get_macaddr, sdk_wifi_set_macaddr.
Also code for sdk_wifi_station_get_connect_status.
Also code for wifi_get_sleep_type and set_sleep_type, noting wifi_set_sleep_type returns a bool success flag, and implement wifi_get_sleep_type using sdk_pm_get_sleep_type.
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.
