Build image for super partition

... for bootstrapping / initializing the device.
Image is built to $(PRODUCT_OUT)/super.img when running
`m dist`. For A/B devices, the image contains other
partitions in the _a slot.

Change-Id: I1459d62f02b95f142dfb3b7608f88ec6801dbf37
Fixes: 111758129
Test: m superimage -j
diff --git a/core/main.mk b/core/main.mk
index a991d88..de23e49 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1190,6 +1190,9 @@
 .PHONY: systemotherimage
 systemotherimage: $(INSTALLED_SYSTEMOTHERIMAGE_TARGET)
 
+.PHONY: superimage
+superimage: $(INSTALLED_SUPERIMAGE_TARGET)
+
 .PHONY: bootimage
 bootimage: $(INSTALLED_BOOTIMAGE_TARGET)