Catch nonexistent BOARD_SEPOLICY_UNION policy files.

Added a new check to make sure that all listed
BOARD_SEPOLICY_UNION files are located somewhere
in the listed BOARD_SEPOLICY_DIRS locations. The
build will error out otherwise.

Change-Id: Icc5febc5fe5a7cccb90ac5b83e6289c2aa5bf069
Signed-off-by: rpcraig <rpcraig@tycho.ncsc.mil>