Resize root partition for kernel 4.14

[neta540]

Not tested!

The code is similar to the method I've done in a private Rpi3b+ Linux Alpine image. Follow up to: Issue https://github.com/DieterReuter/image-builder-rpi64/issues/89 where partitions are no longer resized properly.

[Magnitus-]

I can confirm that the fix worked empirically for me. I tested it on a raspberry pi 3 B with the following hypriot-kit configuration:

{
    "base_image": {
        "image": "hypriot/image-builder:latest",
        "repo": "https://github.com/hypriot/image-builder.git"
    },
    "root_fs": {
        "image": "hypriot/root-fs:latest",
        "repo": "https://github.com/hypriot/os-rootfs.git",
        "target": "arm64-debian",
        "hostname": "black-pearl",
        "groupname": "pirate",
        "username": "pirate",
        "password": "hypriot",
        "version": "v1.2.5"
    },
    "blank_image": {
        "image": "hypriot/blank-image:latest",
        "repo": "https://github.com/hypriot/image-builder-raw.git",
        "device": "rpi"
    },
    "rpi_bootloader": {
        "image": "hypriot/rpi-bootloader:latest",
        "repo": "https://github.com/DieterReuter/rpi-bootloader.git",
        "firmware_repo": "https://www.github.com/raspberrypi/firmware"
    },
    "rpi_kernel": {
        "image": "hypriot/rpi-kernel:latest",
        "repo": "https://github.com/DieterReuter/rpi64-kernel.git",
        "kernel_repo": "https://www.github.com/raspberrypi/linux",
        "kernel_branch": "rpi-4.14.y"
    },
    "rpi_sd_image": {
        "image": "hypriot/rpi-sd-image:latest",
        "repo": "https://github.com/neta540/image-builder-rpi64.git",
        "branch": "update-resize-for-kernel-4.14",
        "docker_engine_version": "18.04.0~ce~3",
        "docker_compose_version": "1.21.1",
        "docker_machine_version": "0.14.0",
        "version": "latest"
    }
}

[DieterReuter]

LGTM, will merge it finally. :stir