Merge branch 'feature/fota_ops_api' into 'master'

Merge branch 'feature/fota_ops_api' into 'master'

Add fota ops api

add ota core function implement
1. esp_ota_begin: init ota partition need to be update, output ota handle,every partition need ota should be init firstly
2. esp_ota_write: write image data to update partition, will record wrote size automatically
3. esp_ota_end: verify image after ota ended
4. esp_ota_set_boot_partition: set next boot partition
5. esp_ota_get_boot_partition: get current running partition

See merge request !256