Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - Merge "build/install: Add USB boot option"
  - 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>

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 2e6b10e..64af432 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 2e6b10e3767fd476b288415949b9d049174e00cc
+Subproject commit 64af432a852c0c0f6b5afc11ddde7609932704db