Automatic merge from submit-queue

Cross-build hyperkube and debian-iptables for ARM. Also add a flannel image

We have to be able to build complex docker images too on `amd64` hosts.
Right now we can't build Dockerfiles with `RUN` commands when building for other architectures e.g. ARM.

Resin has a tutorial about this here: https://resin.io/blog/building-arm-containers-on-any-x86-machine-even-dockerhub/
But it's a bit clumsy syntax.

The other alternative would be running this command in a Makefile:
```
# This registers in the kernel that ARM binaries should be run by /usr/bin/qemu-{ARCH}-static
docker run --rm --privileged multiarch/qemu-user-static:register --reset
```
and 
```
ADD https://github.com/multiarch/qemu-user-static/releases/download/v2.5.0/x86_64_qemu-arm-static.tar.xz /usr/bin
```
Then the kernel will be able to differ ARM binaries from amd64. When it finds a ARM binary, it will invoke `/usr/bin/qemu-arm-static` first and lets `qemu` translate the ARM syscalls to amd64 ones.
Some code here: https://github.com/multiarch

WDYT is the best approach? If registering `binfmt_misc` in the kernels of the machines is OK, then I think we should go with that.
Otherwise, we'll have to wait for resin's patch to be merged into mainline qemu before we may use the code I have here now.

@fgrzadkowski @david-mcmahon @brendandburns @zmerlynn @ixdy @ihmccreery @thockin