axolotl-shiftos: set VENDOR_SECURITY_PATCH to PLATFORM_SECURITY_PATCH

As we are building the stock vendor with shiftos, make them match.

Change-Id: Ida4bb5129b1f5244467636bf338dbe0c4b03e887
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>
diff --git a/BoardConfig.mk b/BoardConfig.mk
index cc6a7b5..d3522f4 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -14,3 +14,6 @@
         TARGET_KERNEL_CONFIG := axolotl_user_defconfig
     endif
 endif
+
+# Security patch level
+VENDOR_SECURITY_PATCH := $(PLATFORM_SECURITY_PATCH)