support multiple boot.img files in release tools

Support for generating multiple boot.img files is required in the
release tools to enable GKI distribution/signing.

Bug: 151094943
Change-Id: I536a286d3123f35918106a52c49b1148d746370f
diff --git a/core/Makefile b/core/Makefile
index 5b3c4c9..a158c9d 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -4167,6 +4167,8 @@
 endif
 ifeq ($(INSTALLED_BOOTIMAGE_TARGET),)
 	$(hide) echo "no_boot=true" >> $@
+else
+	echo "boot_images=$(foreach b,$(INSTALLED_BOOTIMAGE_TARGET),$(notdir $(b)))" >> $@
 endif
 ifneq ($(INSTALLED_VENDOR_BOOTIMAGE_TARGET),)
 	echo "vendor_boot=true" >> $@