Add BOARD_USES_RECOVERY_AS_BOOT to readonly list.

Test: builds
Change-Id: I391e0d168c79e84c1a2acd4fe693e3ed9fbddb53
diff --git a/core/board_config.mk b/core/board_config.mk
index b9b4dc2..f5c8ea5 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -26,6 +26,7 @@
   BOARD_KERNEL_CMDLINE \
   BOARD_KERNEL_BASE \
   BOARD_USES_GENERIC_AUDIO \
+  BOARD_USES_RECOVERY_AS_BOOT \
   BOARD_VENDOR_USE_AKMD \
   BOARD_WPA_SUPPLICANT_DRIVER \
   BOARD_WLAN_DEVICE \