fastboot: add a wipe-super command.

Usage: fastboot wipe-super [super_empty.img]

This command will read the given super_empty.img (using the default one
in ANDROID_PRODUCT_OUT if not specified), and flash by generating a
temporary super.img with no partition data. This command will even work
on retrofit devices.

This command is intended to be used either during device bringup or with
scripts that will manually flash individual dynamic partitions, in place
of using "fastboot flashall".

Bug: 136282057
Test: fastboot wipe-super on retrofit and non-retrofit device
Change-Id: Icab368a63ff36fcce9ac9304eb3966dd38bd78c4