[mender-convert] Research using ARMv6 as target

mender-convert currently on master branch cannot create image for raspberrypi0wifi. It's due to the fact that cpu on this board is armv6 and binaries compiled with armv7 (ubuntu default or others common like linaro) crashes on target.

Because there are customers who would like to use also this board with mender I've created hack with private repo

Best would be to have support for rpi0w on mender-convert. To achieve that few steps will be necessary:

1. use bootlin 7.3.1 toolchain for armv6 (raspberrypi toolchain is 4.8.3 and with this toolchain we cannot compile u-boot > 2018.03 due strict requirement on gcc)
2. install this toolchain during docker container creation
3. use this toolchain for u-boot compilation + tools

Issue with this approach is that mender client is compiled during docker container creation and we don't have device specific parameter to be able to distinguish with which toolchain build mender-client.
Issue is more general as if we would like to support e.g. aarch64 board we have same issue.

Acceptance criteria:

• perform following research steps for further analysis:
  • adjust mender-convert tooling to target ARMv6 (Mender client, UBoot, GRUB, etc.)
    • 3rd-party toolchain (Linaro/bootlin) must be used in both cases: host conversion & conversion in docker
  • test resulting image on RPi0, RPi3 and BBB

From this we can learn about any obstacles and further version-mix complexities to see if there is a sane path forward here.