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)

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 937d9d8..9e78ba7 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 937d9d8cf17739f670dcf40528bdc59d70a3349b
+Subproject commit 9e78ba77a04219512bae5b0855b573ab03b6c1a5