Rename INSTALLED_SYSTEMIMAGE to INSTALLED_SYSTEMIMAGE_TARGET.

This gives a consistent mapping between partition names and the image
targets (i.e. partition => INSTALLED_PARTITIONIMAGE_TARGET). This CL
also creates an alias for the old name so that device-specific Makefiles
that refer to INSTALLED_SYSTEMIMAGE would keep working.

This CL renames image-for-partitions to images-for-partitions, and
returns INSTALLED_SYSTEMIMAGE_TARGET for system. It doesn't change the
actual functionality, but allows substantially simplifying the rule.

Test: `m dist` with aosp_taimen-userdebug
Test: `m dist` with a target that uses super partition
Change-Id: I2954ee5d855c1652a33c640ee615649588d5bc04
diff --git a/core/main.mk b/core/main.mk
index 5eebe1e..702c6a5 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1354,7 +1354,7 @@
   endif
 
   ifeq ($(EMMA_INSTRUMENT),true)
-    $(JACOCO_REPORT_CLASSES_ALL) : $(INSTALLED_SYSTEMIMAGE)
+    $(JACOCO_REPORT_CLASSES_ALL) : $(INSTALLED_SYSTEMIMAGE_TARGET)
     $(call dist-for-goals, dist_files, $(JACOCO_REPORT_CLASSES_ALL))
 
     # Put XML formatted API files in the dist dir.