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