Drop BOARD_SEPOLICY_UNION.

As suggested in the comments on
https://android-review.googlesource.com/#/c/141560/


drop BOARD_SEPOLICY_UNION and simplify the build_policy logic.
Union all files found under BOARD_SEPOLICY_DIRS.

Unlike BOARD_SEPOLICY_REPLACE/IGNORE, on which we trigger an error
to catch any lingering uses and force updating of the BoardConfig.mk
files, we only warn on uses of BOARD_SEPOLICY_UNION to avoid
breaking the build until all device BoardConfig*.mk files have been
updated, and since they should be harmless - the files will be unioned
regardless.

Change-Id: I4214893c999c23631f5456cb1b8edd59771ef13b
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>