soong_config: Support SYSTEM_EXT* sepolicy vars

The new variable name reflects its actual usage.

Keep compatibility with BOARD_PLAT_* because it has been a
convention for years. Also add warning messages for BOARD_PLAT_*
variables via KATI_deprecated_var.

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 Elsner <google@ix5.org>
Change-Id: I58c64839cc513ae082cd3ee3c1e108843ea7439e