Update git submodules

* Update docs/submodules/sfc from branch 'stable/danube'
  - Remove references to older releases on documentation

     - Rename all Brahmaputra/Colorado links to Danube
     - Bumped OvS version from 2.5.90 to 2.6.1
     - Bumped ODL version from Boron to Boron SR2
     - Changed release date

    Change-Id: I4286cbd074b377b4b33e25f33646bb05075f10f5
    Signed-off-by: Juan Vidal <juan.vidal.allende@ericsson.com>
    (cherry picked from commit e61239dbb4f96bb4b798cd3c6e6d6c953d4fcee4)

diff --git a/docs/submodules/sfc b/docs/submodules/sfc
index 6a9c9e6..a6f4008 160000 (submodule)
--- a/docs/submodules/sfc
+++ b/docs/submodules/sfc
@@ -1 +1 @@
-Subproject commit 6a9c9e69da60996ed3b738dd71b744df61cec9d7
+Subproject commit a6f40087fd15131c50d5cf3594d0ef702cdc40db