Android.mk: Support SYSTEM_EXT* sepolicy

The new variable name reflects its actual usage.

Keep compatibility with BOARD_PLAT_* because it has been a
convention for years.

Amend the README to document the new variables.

Test: `make selinux_policy` with
      `SYSTEM_EXT_{PUBLIC,PRIVATE}_SEPOLICY_DIRS` set,
      observe additions in `$(TARGET_COPY_OUT_SYSTEM_EXT)/etc/selinux`

Signed-off-by: Felix <google@ix5.org>
Change-Id: If8188feb365eb9e500f2270241fa190a20e9de01
2 files changed