Update git submodules

Update git submodules

* Update docs/submodules/fuel from branch 'stable/danube'
  - build/install: Add USB boot option

    Use isohybrid to add MBR for booting via BIOS
    from USB flash drives

    Also, Makefile "$(wildcard ubuntu-builder/*)" does not expand
    to what is expected (does not include ubuntu-builder/Dockerfile),
    as it uses a previous cached value.
    Check this link for an explanation:
    http://stackoverflow.com/questions/41470676/makefile-variable-expansion-with-wildcard-when-rule-creates-new-file

    JIRA: FUEL-236

    Change-Id: I9c6521a382917822ce930ddf76391c5f3c3a9cb7
    Signed-off-by: Matei Valeanu <Matei.Valeanu@enea.com>
    (cherry picked from commit eb8fbde87578e3853ffd23e403fd46bac6039cd0)