We currently don't have an example on that particular feature. I assume it's a single chip setup ? You want the bootloader to read the SDcard and update the application firmware on the same chip ? We only have example for 2 chip solution where the image sent from one chip/PC to the nRF52. You would need to write your own code to read the SD card and store the image to flash. You can reuse some part of the bootloader, for example flash storing, init packet checking, image swapping.
