image-ktn-sparkle: Do not overwrite SRC_URI
Other included files like image-ktn-swupdate-platform.inc for swupdate config also use SRC_URI. Append instead of overwrite the variable so the content already set is kept.
Other included files like image-ktn-swupdate-platform.inc for swupdate config also use SRC_URI. Append instead of overwrite the variable so the content already set is kept.