RFC: arm64 support [ci-skip] [not for merge]

[mor1]

This PR is not for merge but is the starting point for ARM64 support.
It has now successfully built a kernel and initrd that have booted a packet.net ARM64 box.

[AkihiroSuda]

Can we deduplicate kernel/Dockerfile,Makefile,kernel_config across all the platforms?

i.e.

• kernel/Dockerfile: universal Dockerfile, accepts specifying ARCH via build-arg
• Makefile: ditto via make ARCH=...
• kernel_config: common parts across all the archs
• kernel_config.{x86_64,arm64,...}: architecture-specific part. Concatinated to kernel_config in the build container

[rn]

@AkihiroSuda hopefully today I'll tidy up the kernel build a little and that might make it easier to dedup

[justincormack]

you should not be setting this unconditionally, you should use uname to find it

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "project-arm64" git@github.com:mor1/linuxkit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354472624
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[zlim]

cc @arm64b

[justincormack]

Going to close this as parts of arm64 are now being merged in and this does not apply.