Flashkernel install fails for arm64 platform on chroot env

[Peter Rudenko]

disk-image-create -x -x -a arm64 \
     ubuntu ironic-python-agent-ramdisk -o image

> /etc/kernel/postinst.d/dkms:
> * dkms: running auto installation service for kernel 5.4.0-113-generic
> ...done.
> /etc/kernel/postinst.d/initramfs-tools:
> update-initramfs: Generating /boot/initrd.img-5.4.0-113-generic
> cryptsetup: WARNING: Couldn't determine root device
> Label cloudimg-rootfs not found in /dev/disk/by-label
> Warning: root device LABEL=cloudimg-rootfs does not exist
> Unsupported ioctl: cmd=0x5331
> Unsupported ioctl: cmd=0x5331
> Unsupported ioctl: cmd=0x5331
> Unsupported ioctl: cmd=0x5331
> Unsupported ioctl: cmd=0x5331
> Unsupported ioctl: cmd=0x5331
> Unsupported ioctl: cmd=0x5331
> Unsupported ioctl: cmd=0x5331
> Unsupported ioctl: cmd=0x5331
> Unsupported ioctl: cmd=0x5331
> W: Couldn't identify type of root file system for fsck hook
> W: plymouth module (/usr/lib/aarch64-linux-gnu/plymouth//.so) missing, skipping that theme.
> flash-kernel: deferring update (trigger activated)
> /etc/kernel/postinst.d/zz-flash-kernel:
> flash-kernel: deferring update (trigger activated)
> Processing triggers for flash-kernel (3.103ubuntu1~20.04.4) ...
> Unsupported platform.
> dpkg: error processing package flash-kernel (--configure):
> installed flash-kernel package post-installation script subprocess returned error exit status 1
> Errors were encountered while processing:
> flash-kernel
> E: Sub-process /usr/bin/dpkg returned an error code (1)
returncode: 100
install-packages failed with returncode 100

Here's a related issues
https://bugs.launchpad.net/ubuntu/+source/flash-kernel/+bug/623276
https://github.com/tolstoyevsky/pieman/issues/85
https://github.com/tolstoyevsky/pieman/pull/89/files