This change adds the support of odm sepolicy customization, which can
be configured through the newly added build varaible:
    - BOARD_ODM_SEPOLICY_DIRS += device/${ODM_NAME}/${BOM_NAME}/sepolicy

Also moving precompiled sepolicy to /odm when BOARD_ODM_SEPOLICY_DIRS
is set. On a DUT, precompiled sepolicy on /odm will override the one in
/vendor. This is intentional because /odm is the hardware customization
for /vendor and both should be updated together if desired.

Bug: 64240127
Test: boot a device with /odm partition
Change-Id: Ia8f81a78c88cbfefb3ff19e2ccd2648da6284d09