Update git submodules

Update git submodules

* Update docs/submodules/armband from branch 'stable/euphrates'
  - Merge "reclass: system: Pin Armband apt-mk to nightly ver" into stable/euphrates
  - reclass: system: Pin Armband apt-mk to nightly ver

    Recently, Mirantis apt-mk repos were changed upstream from using the
    'nightly' dist to 'stable'. However, Armband only provides the
    'nightly' dist for now, so hardcode our repo definitions to that.

    Change-Id: I43b7531905b1bbe2a79a3fb2215dea9c35932b7c
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
    (cherry picked from commit ada64bbcb86666e792e72413ea2900e8ff60c4b3)