Add better per-device sepolicy support.

This is a rewrite of the existing implementation.
Three new variables are now needed to add/modify
the exisitng base policy. They are, BOARD_SEPOLICY_REPLACE
and BOARD_SEPOLICY_UNION which govern what files
are replaced and concatenated, and BOARD_SEPOLICY_DIRS
which lists the various directories that will contain
the BOARD_SEPOLICY_REPLACE and BOARD_SEPOLICY_UNION
policy files.

Change-Id: Id33381268cef03245c56bc5242fec7da9b6c6493
Signed-off-by: rpcraig <robertpcraig@gmail.com>