Introduce BOARD_KERNEL_BINARIES to soong

Bug: 161563386
Test: use it

Change-Id: I8ff429a329a9aa2a8ab6c8b4501356a84a7aead6
diff --git a/android/variable.go b/android/variable.go
index 5826138..c1e1b42 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -344,6 +344,8 @@
 	InstallExtraFlattenedApexes *bool `json:",omitempty"`
 
 	BoardUsesRecoveryAsBoot *bool `json:",omitempty"`
+
+	BoardKernelBinaries []string `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {