Get build.prop for init_boot.img from the ramdisk props

boot.img and init_boot.img get their build properties from the platform
ramdisk properties.

Test: "m dist" then drop the OTA zip file into
android.github.io/analyseOTA to verify init_boot is present.
Bug: 211823573

Change-Id: I411e62bf0fc3cb49d4b12dadb52d9820fca041fa
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 8506199..60a9265 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -498,8 +498,9 @@
   def GetPartitionBuildProp(self, prop, partition):
     """Returns the inquired build property for the provided partition."""
 
-    # Boot image uses ro.[product.]bootimage instead of boot.
-    prop_partition = "bootimage" if partition == "boot" else partition
+    # Boot image and init_boot image uses ro.[product.]bootimage instead of boot.
+    # This comes from the generic ramdisk
+    prop_partition = "bootimage" if partition == "boot" or partition == "init_boot" else partition
 
     # If provided a partition for this property, only look within that
     # partition's build.prop.