Replace the `generic_x86_64-ext` device type with the `generic-amd64`

[alexgg]

We have two different device families for x86_64 device types, https://github.com/balena-os/balena-intel and https://github.com/balena-os/balena-generic.

We want balena-generic to become the home for UEFI device types, both ARM and x86_64. This is currently the space or servers but we see UEFI as an emerging standard.

The balena-intel repository will remain the home of closed hardware, embedded x86_64 device types, including the NUC. As of today, the generic_x86-64-ext" machine is an extended version of the NUC that is used for x86_64 servers and workstation. We want this device type to be replaced with thegeneric-amd64`, while keeping the same cloud slug. Hence, these two devices need to be made equivalent.

[jellyfish-bot]

[alexgg] This issue has attached support thread https://jel.ly.fish/d2c1a621-4a94-4b44-ae2d-9b0338e31932

[jellyfish-bot]

[alexgg] This issue has attached support thread https://jel.ly.fish/c756a0f9-2eed-48fb-bd23-becab5ec0750

[jellyfish-bot]

[alexgg] This issue has attached support thread https://jel.ly.fish/64f85fc0-167f-40c2-838c-953eda5eac8c

[alexgg]

An arch call decision was made to add MBR boot support to the generic-amd64 image so it is a drop-in replacement for the `genericx86-64-ext" one.

[alexgg]

For the time being we will keep both the Generic x86-64 (legacy MBR) targeting older BIOS some industrial PC type of devices, and the Generix x86-64 (GPT) one which targets server like UEFI systems.

[jellyfish-bot]

[alexgg] This has attached https://jel.ly.fish/b718baa7-dbc3-4490-87e3-b597c5653dd5

[alexgg]

What's left is to document in the balena-intel README the different type of images and their target devices,