community/heimdall: update to 2.1.0

Deprecate heimdall_flash_kernel.sh in favour of new script heimdall_flash_isorec_kernel.sh which handles vmlinuz+dtb concatenation, and deprecate heimdall_wait_for_device.sh script as heimdall 2.1.0 has a --wait argument that can be used instead. Both heimdall_flash_kernel.sh and heimdall_wait_for_device.sh will be removed from the package in a future update.

For full changelog, see https://git.sr.ht/~grimler/Heimdall/refs/v2.1.0

The release includes one segfault fix for when using a certain combination of options that I will backport to heimdall 2.0.2 in 3.20 branch in a separate MR.