Upgrade from 18.04 LTS to 20.04 LTS on RasPi 4

[Michael Wind]

The error with upgrading the flash-kernel existing since half a year or longer...

ProblemType: Package
DistroRelease: Ubuntu 18.04
Package: flash-kernel 3.98ubuntu11~18.04.2
Uname: Linux 4.19.81-v8-20 aarch64
ApportVersion: 2.20.9-0ubuntu7.16
Architecture: arm64
Date: Sat Aug 22 11:59:47 2020
ErrorMessage: installed flash-kernel package post-installation script subprocess returned error exit status 1
Python3Details: /usr/bin/python3.6, Python 3.6.9, python3-minimal, 3.6.7-1~18.04
PythonDetails: /usr/bin/python2.7, Python 2.7.17, python-minimal, 2.7.15-3~oibaf~b
RelatedPackageVersions:
 dpkg 1.19.0.5ubuntu2.3
 apt 1.6.12ubuntu0.1
SourcePackage: flash-kernel
Title: package flash-kernel 3.98ubuntu11~18.04.2 failed to install/upgrade: installed flash-kernel package post-installation script subprocess returned error exit status 1
UpgradeStatus: Upgraded to bionic on 2020-08-22 (0 days ago)
modified.conffile..etc.initramfs.post-update.d.flash-kernel: [deleted]
modified.conffile..etc.kernel.postinst.d.zz-flash-kernel: [deleted]
modified.conffile..etc.kernel.postrm.d.zz-flash-kernel: [deleted]