Update git submodules

Update git submodules

* Update docs/submodules/netconf from branch 'master'
  to 51697b1224a8a84766d5139916ecc0f1889227dc
  - Merge "NETCONF-241: Switch to using Exificient"
  - NETCONF-241: Switch to using Exificient

    Exificient has a bit nicer API, it is available from maven central and
    has a friendly (MIT) license, plus the source code is on GitHub. Switch
    to using it instead of OpenEXI.

    Change-Id: I2cb7bf244361b021afb4cdc275e2442f5b778056
    Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
    Signed-off-by: Andrej Mak <andrej.mak@pantheon.tech>