Add a target for building kernel with buildx (#3792)

* Use latest kernel in linuxkit

Signed-off-by: Frédéric Dalleau <frederic.dalleau@docker.com>

* Parallelize kernel source compression

This surpringly saves a lot of time:
M1: from 340 to 90 seconds
Intel: from 527 to 222 seconds (2 cores 4 threads)

Signed-off-by: Frédéric Dalleau <frederic.dalleau@docker.com>

* Add buildx target

buildx can use remote builders and automatically generate the multiarch manifest.
A properly configured builder is required :

First create docker context for the remote builders :
$ docker context create node-<arch> --docker "host=ssh://<user>@<host>"

Then create a buildx configuration using the remote builders:
$ docker buildx create --name kernel_builder --platform linux/amd64
$ docker buildx create --name kernel_builder --node node-arm64 --platform linux/arm64 --append
$ docker buildx use kernel_builder
$ docker buildx ls

Signed-off-by: Frédéric Dalleau <frederic.dalleau@docker.com>

* Add a PLATFORMS variable to declare platforms needed for buildx

Signed-off-by: Frédéric Dalleau <frederic.dalleau@docker.com>

* Make image name customizable

Signed-off-by: Frédéric Dalleau <frederic.dalleau@docker.com>

* Do not tag use the architecture suffix for images built with buildx

Signed-off-by: Frédéric Dalleau <frederic.dalleau@docker.com>

* Add make kconfigx to upgrade configs using buildx

To update configuration for 5.10 kernels use :
make -C kernel KERNEL_VERSIONS=5.10.104 kconfigx

Signed-off-by: Frédéric Dalleau <frederic.dalleau@docker.com>

---------

Signed-off-by: Frédéric Dalleau <frederic.dalleau@docker.com>

linuxkit.yml

@@ -1,5 +1,5 @@
 kernel:
-  image: linuxkit/kernel:5.10.104
+  image: linuxkit/kernel:5.15.27
   cmdline: "console=tty0 console=ttyS0 console=ttyAMA0"
 init:
   - linuxkit/init:14df799bb3b9e0eb0491da9fda7f32a108a2e2a5