Add BOARD_KERNEL_BINARIES.

Test: use it
Bug: 161563386
Change-Id: Id07dffe7d8ef5122c7cf337b29fecba128fc7640
diff --git a/core/soong_config.mk b/core/soong_config.mk
index c7bf61e..efd811e 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -215,6 +215,8 @@
 
 $(call add_json_bool, BoardUsesRecoveryAsBoot, $(BOARD_USES_RECOVERY_AS_BOOT))
 
+$(call add_json_list, BoardKernelBinaries, $(BOARD_KERNEL_BINARIES))
+
 $(call json_end)
 
 $(file >$(SOONG_VARIABLES).tmp,$(json_contents))