added: esp_clk_cpu_freq esp_set_cpu_freq esp_deep_sleep

[Jos-Ven]

esp_set_cpu_freq To change the clock frequency of the CPU.
esp_clk_cpu_freq To check the new clock frequency.
esp_deep_sleep enable To enter a deep sleep mode for a specified time.

I use these calls to reduce the power usage.

[MitchBradley]

Is this ready to merge?

[Jos-Ven]

Hi Mitch,

I tried to submit my changes to you.
If all went well there should be additions in the files
interface.h and extend.c
Added files:
app/esp8266-rtos/tasking_rtos.fth
app/esp8266-rtos/tests/pe_tasking.fth
app/esp8266-rtos/tests/spi.fth
app/esp8266-rtos/tests/spi_ledstrip_apa201.fth

Kind regards, Jos