Build image for clean super partition.

This adds a new super_empty.img output to the build when
PRODUCT_USE_LOGICAL_PARTITIONS is true and a super partition size is
set. This will be used by fastboot flashall in fastbootd, since it needs
a record of the partition layout in case the super partition is corrupt
or not yet flashed.

The full super.img (which contains embedded partition data) is still
available via "make dist".

Bug: 78793464
Test: super_empty.img generated when PRODUCT_USE_LOGICAL_PARTITIONS is
      true

Change-Id: I9f00b07e5b685a3230803fa1a5a569544d0ab625