Commit Icc5febc5fe5a7cccb90ac5b83e6289c2aa5bf069
introduced a new error check for non existent
BOARD_SEPOLICY_UNION files. Need an update to
the docs describing the change.

Change-Id: If96c9046565b05e0811ab2d526ae12a3b8b90bf0
Signed-off-by: rpcraig <rpcraig@tycho.ncsc.mil>