Failed to build firmware for arm64 because packages/linux-kernel/build-linux-firmware.sh ignores LOCALVERSION.
packages/linux-kernel/build-kernel.sh generates versions like 6.1.4-v8-arm64-vyos, while build-linux-firmware.sh searches for 6.1.4-arm64-vyos.
packages/linux-kernel/kernel-vars contains only KERNEL_VERSION=6.1.4 and KERNEL_SUFFIX=-arm64-vyos. To pass LOCALVERSION to build-linux-firmware.sh, it may be necessary to include -v8 in kernel-vars somehow.