flashers: remove need for heimdall_wait_for_device.sh and fix heimdall-isorec flash

These changes depend on heimdall 2.1.0: https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/69089.

Heimdall 2.1.0 has a new option --wait, which can be used instead of running heimdall_wait_for_device.sh, so this MR simplifies the flash methods accordingly.

We also make use of an update flash script heimdall_flash_isorec_kernel.sh, which takes a fifth argument with device dtb, and use that one for heimdall-isorec flashes. This fixes https://gitlab.com/postmarketOS/boot-deploy/-/issues/26.