Update git submodules

Update git submodules

* Update docs/submodules/snaps from branch 'master'
  - Merge "Check for kernel_url and ramdisk_url keys in dict"
  - Check for kernel_url and ramdisk_url keys in dict

    Need to check if the above keys exist in image_metadata
    before checking for their assigned values in case
    they are not declared (e.g. obsolete when creating a
    one part image)

    Change-Id: I22fa259b320a425d6ad030eeb9bf68b4c06b667f
    Signed-off-by: Catalina Focsa <catalina.focsa@enea.com>