Enforce modern version of cmake

[mrquincle]

The cmake build tool is versatile, but the syntax is ugly. We're supporting quite an old cmake version. Many files from Zephyr (which we will be relevant for supporting Matter on the nRF52840) state:

cmake_minimum_required(VERSION 3.20.0)

Hence, we shouldn't stick to the cmake version supplied with an operating system. For example, if you're on LTS for Ubuntu, you'll be at 20.04 which has cmake version 3.16.3: https://launchpad.net/ubuntu/focal/+source/cmake. You can however easily update to a more modern version of cmake without having to compile anything yourselves. Check the instructions yourself on https://apt.kitware.com/.

sudo apt remove cmake
wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | sudo tee /usr/share/keyrings/kitware-archive-keyring.gpg >/dev/null
echo 'deb [signed-by=/usr/share/keyrings/kitware-archive-keyring.gpg] https://apt.kitware.com/ubuntu/ focal main' | sudo tee /etc/apt/sources.list.d/kitware.list >/dev/null
sudo apt update
sudo rm /usr/share/keyrings/kitware-archive-keyring.gpg
sudo apt install kitware-archive-keyring
sudo apt install cmake

If we bump the cmake version we can for example remove a lot of the calculations around hexadecimal values (which is a moderately recent addition to cmake).

If we do so, we of course have to instruct people on how to get a more modern version of cmake on their system. When we've described this properly it's time to enforce it.

[mrquincle]

This can remove all from_hex and to_hex helper functions.

[vliedel]

An Ubuntu LTS is supported for 5 years, so 18.04 is still a valid version. However, it's quite simple indeed to upgrade the cmake version, so i'm ok with bumping the requirement to 3.20.

[mrquincle]

This is part of branch https://github.com/crownstone/bluenet/tree/cmake-and-runtime but PR will only be opened after acceptance of current open ones.