imagetool: make the image_save_datafile() available to all image types

Move the image_save_datafile() function from an U-Multi specific file
(default_image.c) to a file common to all image types (image.c). And rename it
to genimg_save_datafile(), to make clear it is useful for any image type.

Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>