Passing hard coded kernel image's file name to boot-deploy caused boot-deploy to generate broken grub/extlinux config for linux-edge.

postmarketos-mkinitfs always pass a hard-coded kernel image's file name (vmlinuz) to boot-deploy. This has caused boot-deploy to always generate grub/extlinux config with the wrong kernel image's file name when the kernel image's file name isnt vmlinuz. (linux-edge kernel image's file name is vmlinuz-edge)