fngsystem: Fixed copy of boot files for i.MX8M

The i.MX8M uses the meta-freescale layer. The layer adds the
KERNEL_IMAGETYPES to the IMAGE_BOOT_FILES. This breaks the copy
operation in the do_collect_files function because we don't handle
the kernel together with the boot files.
The issue is fixed by creating a copy of the IMAGE_BOOT_FILES, removing
the KERNEL_IMAGETYPES and using the copy in the function.