Build boot image if BOARD_KERNEL*_BOOTIMAGE_PARTITION_SIZE

Also gets the correct partition size and checks correctly.

Test: build aosp_arm64

Change-Id: I004e4b04d225d72db1904b983a6d03a916406085
diff --git a/core/board_config.mk b/core/board_config.mk
index 1ab96ea..eeea49b 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -363,6 +363,8 @@
     BUILDING_BOOT_IMAGE :=
   else ifdef BOARD_BOOTIMAGE_PARTITION_SIZE
     BUILDING_BOOT_IMAGE := true
+  else ifneq (,$(foreach kernel,$(BOARD_KERNEL_BINARIES),$(BOARD_$(call to-upper,$(kernel))_BOOTIMAGE_PARTITION_SIZE)))
+    BUILDING_BOOT_IMAGE := true
   endif
 else ifeq ($(PRODUCT_BUILD_BOOT_IMAGE),true)
   ifeq ($(BOARD_USES_RECOVERY_AS_BOOT),true)