This platform copies more files into `/boot` which are necessary to be
able to boot IoT on some single board computers.
We also immediately set this on the `Aarch64_IoT` platform which needs
u-boot to be placed in the `/boot`.
This closes#3312.
GCE requires an archive with specific (tar) options, so we should have a
platform format defined separately for it.
Add the GCE image types to the x86_64 arch using the new platform
format.
These objects describes the hardware an image runs on. Including
- architecture
- bootloader
- required firmware
Use the platform abstraction to move firmware packages out of the package set
definitions.
