Refactor sepolicy version related codes

1. Move BOARD_SEPOLICY_VERS to build/make/core/config.mk where
PLATFORM_SEPOLICY_VERSION is set.

2. Remove hard-coded versions for the treble tests.

Test: build
Change-Id: Iab338a1f16855205d62312d4247a15ea766be1fe
diff --git a/core/config.mk b/core/config.mk
index 4794816..bfff84e 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -831,6 +831,15 @@
 sepolicy_major_vers :=
 sepolicy_minor_vers :=
 
+# BOARD_SEPOLICY_VERS must take the format "NN.m" and contain the sepolicy
+# version identifier corresponding to the sepolicy on which the non-platform
+# policy is to be based. If unspecified, this will build against the current
+# public platform policy in tree
+ifndef BOARD_SEPOLICY_VERS
+# The default platform policy version.
+BOARD_SEPOLICY_VERS := $(PLATFORM_SEPOLICY_VERSION)
+endif
+
 # A list of SEPolicy versions, besides PLATFORM_SEPOLICY_VERSION, that the framework supports.
 PLATFORM_SEPOLICY_COMPAT_VERSIONS := \
     28.0 \