Policy Generation:

Additional, per device, policy files can be added into the
policy build.

They can be configured through the use of three variables,
they are:
1. BOARD_SEPOLICY_REPLACE
2. BOARD_SEPOLICY_UNION
3. BOARD_SEPOLICY_DIRS
4. BOARD_SEPOLICY_IGNORE

The variables should be set in the BoardConfig.mk file in
the device or vendor directories.

BOARD_SEPOLICY_UNION is a list of files that will be
"unioned", IE concatenated, at the END of their respective
file in external/sepolicy. Note, to add a unique file you
would use this variable.

BOARD_SEPOLICY_REPLACE is a list of files that will be
used instead of the corresponding file in external/sepolicy.

BOARD_SEPOLICY_DIRS contains a list of directories to search
for BOARD_SEPOLICY_UNION and BOARD_SEPOLICY_REPLACE files. Order
matters in this list.
eg.) If you have BOARD_SEPOLICY_UNION := widget.te and have 2
instances of widget.te files on BOARD_SEPOLICY_DIRS search path.
The first one found (at the first search dir containing the file)
gets processed first.
Reviewing out/target/product/<device>/etc/sepolicy_intermediates/policy.conf
will help sort out ordering issues.

It is an error to specify a BOARD_POLICY_REPLACE file that does
not exist in external/sepolicy.

It is an error to specify a BOARD_POLICY_REPLACE file that appears
multiple times on the policy search path defined by BOARD_SEPOLICY_DIRS.
eg.) if you specify shell.te in BOARD_SEPOLICY_REPLACE and
BOARD_SEPOLICY_DIRS is set to
"vendor/widget/common/sepolicy device/widget/x/sepolicy" and shell.te
appears in both locations, it is an error. Unless it is in
BOARD_SEPOLICY_IGNORE to be filtered out. See BOARD_SEPOLICY_IGNORE
for more details.

It is an error to specify the same file name in both
BOARD_POLICY_REPLACE and BOARD_POLICY_UNION.

It is an error to specify a BOARD_SEPOLICY_DIRS that has no entries when
specifying BOARD_SEPOLICY_REPLACE.

BOARD_SEPOLICY_IGNORE is a list of paths (directory + filename) of
files that are not to be included in the resulting policy. This list
is passed to filter-out to remove any paths you may want to ignore. This
is useful if you have numerous config directories that contain a file
and you want to NOT include a particular file in your resulting
policy file, either by UNION or REPLACE.
Eg.) Suppose the follwoing:
     BOARD_SEPOLICY_DIRS := X Y
     BOARD_SEPOLICY_REPLACE := A
     BOARD_SEPOLICY_IGNORE := X/A

     Directories X and Y contain A.

     The resulting policy is created by using Y/A only, thus X/A was
     ignored.

Example BoardConfig.mk Usage:
From the Tuna device BoardConfig.mk, device/samsung/tuna/BoardConfig.mk

BOARD_SEPOLICY_DIRS := \
        device/samsung/tuna/sepolicy

BOARD_SEPOLICY_UNION := \
        genfs_contexts \
        file_contexts \
        sepolicy.te