Add BUILD_BOOT_IMAGE flag

Bug: 123428770
Test: Built cuttlefish and tested in presubmit.
Change-Id: I9f2a4eb6aaf8a326acdd80813dde14ab771c76b2
diff --git a/shared/device.mk b/shared/device.mk
index dbe84dc..08c753c 100644
--- a/shared/device.mk
+++ b/shared/device.mk
@@ -21,7 +21,7 @@
 
 PRODUCT_SHIPPING_API_LEVEL := 29
 PRODUCT_OTA_ENFORCE_VINTF_KERNEL_REQUIREMENTS := false
-
+PRODUCT_BUILD_BOOT_IMAGE := true
 DISABLE_RILD_OEM_HOOK := true
 
 # Properties that are not vendor-specific. These will go in the product