Partition/SPI/OTA docs & OTA new functionality
* Update partition, SPI flash & OTA docs to reflect functionality changes
* Refactor OTA implementation to perform checks mentioned in API doc
* Add new functions to OTA API: esp_ota_get_running_partition() & esp_ota_get_next_update_partition() functions
* Add spi_flash_cache2phys() & spi_flash_phys2cache() functions to support esp_ota_get_running_partition()
See merge request !513