[mender-client] Raspian packages do not support ECC keys

The Raspian packages for the mender-client do not support ECC keys. The reason is that the support for those keys in mendersoftware/openssl is available only if the client is compiled with openssl >= 1.1.1.

This is not the case for our Raspian packages, which are built on stretch, which ships openssl 1.1.0. Consider upgrading the build pipeline to buster, to make it use openssl 1.1.1 instead.

This task is blocked until Debian releases new stable release Bullseye, ETA 14th August 2021

Acceptance criteria:

• Move the Mender dist packages build system to Debian Buster
• Rebuild master package and manually test that ECC keys are supported now