Update git submodules

* Update docs/submodules/releng from branch 'master'
  to e6ac439da0f91fd06169626bf626bbdf160217ba
  - Merge "Reduce disk-image-builder env vars"
  - Reduce disk-image-builder env vars

    Change-Id: I77325ebd1e7bc9cabe4833a9e91541ecb0dd0e3b
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index ffd5e01..e6ac439 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit ffd5e01a7293fd09f7042254eb5533fd0d186329
+Subproject commit e6ac439da0f91fd06169626bf626bbdf160217ba