flashers: heimdall-isorec: use heimdall_flash_isorec_kernel.sh

This wrapper script, which was added in alpinelinux's heimdall 2.1.0
package, replaces heimdall_flash_kernel.sh and takes a fifth arg with
dtb name. The script then concatenates vmlinuz and the dtb into a
vmlinuz-dtb file and flashes it to the kernel partition.

Boot-deploy was previously responsible for creating vmlinuz-dtb, but
to save disk space we have moved away from creating it
unconditionally.

Fixes: https://gitlab.com/postmarketOS/boot-deploy/-/issues/26