Make build support vendor_kernel_boot image

Add vendor_kernel_boot image for vendors whose bootloader support
extra first stage booting kernel modules ramdisks. This benefit
kernel repo to build kernel-artifacts only image without Andorid
artifacts dependency.

Bug: 214409109
Signed-off-by: Lucas Wei <lucaswei@google.com>
Change-Id: If07218b86a7751b3d452a172610af960f5f9ec74
diff --git a/core/product.mk b/core/product.mk
index 032ca6b..4ddc4fe 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -310,6 +310,7 @@
 _product_single_value_vars += PRODUCT_BUILD_INIT_BOOT_IMAGE
 _product_single_value_vars += PRODUCT_BUILD_DEBUG_BOOT_IMAGE
 _product_single_value_vars += PRODUCT_BUILD_VENDOR_BOOT_IMAGE
+_product_single_value_vars += PRODUCT_BUILD_VENDOR_KERNEL_BOOT_IMAGE
 _product_single_value_vars += PRODUCT_BUILD_DEBUG_VENDOR_BOOT_IMAGE
 _product_single_value_vars += PRODUCT_BUILD_VBMETA_IMAGE
 _product_single_value_vars += PRODUCT_BUILD_SUPER_EMPTY_IMAGE