fngsystem: Fixed do_collect_files in fngsystem-image

Since Kirkstone, there is a new way of disabling symlinks of the boot artifacts (kernel, etc.): To disable the creation of these links, the variable KERNEL_IMAGETYPE_SYMLINK can be set to 0. Since we use a FAT boot partition, links in the boot folder of the image lead to errors during the extraction of the image and we have to disable them. However, the new variable also disables the creation of links for the devicetrees and kernel in the deploy folder. To solve errors during the package of Flash-N-Go-System, the copy operations for the boot files and kernel have been changed.