Export BOARD_USES_RECOVERY_AS_BOOT to soong.

Test: pass
Bug: 147347110
Change-Id: I4ba4f626f02c42b1700a971326557abd23da8340
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 7c58082..d223792 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -203,6 +203,8 @@
 
 $(call add_json_bool, InstallExtraFlattenedApexes, $(PRODUCT_INSTALL_EXTRA_FLATTENED_APEXES))
 
+$(call add_json_bool, BoardUsesRecoveryAsBoot, $(BOARD_USES_RECOVERY_AS_BOOT))
+
 $(call json_end)
 
 $(file >$(SOONG_VARIABLES).tmp,$(json_contents))