Fix images-for-partitions() for 'init_boot'

Macro images-for-partitions() returns $(INSTALLED_INIT_BOOTIMAGE_TARGET)
for the 'init_boot' partition, which is empty. It should return
$(INSTALLED_INIT_BOOT_IMAGE_TARGET) instead.

Bug: 203698939
Test: `make` without chaining 'init_boot' partition
Change-Id: I387813283dfad7e1b2f762a5fa4065775977caf7
diff --git a/core/Makefile b/core/Makefile
index 4c1ae51..0c14fd8 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -3546,7 +3546,10 @@
 # $(INSTALLED_VENDORIMAGE_TARGET)" for "system vendor".
 # (1): list of partitions like "system", "vendor" or "system product system_ext".
 define images-for-partitions
-$(strip $(foreach item,$(1),$(if $(filter $(item),system_other),$(INSTALLED_SYSTEMOTHERIMAGE_TARGET),$(INSTALLED_$(call to-upper,$(item))IMAGE_TARGET))))
+$(strip $(foreach item,$(1),\
+  $(if $(filter $(item),system_other),$(INSTALLED_SYSTEMOTHERIMAGE_TARGET),\
+    $(if $(filter $(item),init_boot),$(INSTALLED_INIT_BOOT_IMAGE_TARGET),\
+      $(INSTALLED_$(call to-upper,$(item))IMAGE_TARGET)))))
 endef
 
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