fastboot: Avoid reboots to userspace when using flashall/update.
Reboots to fastbootd (userspace fastboot) take a long time, particularly
due to the orange AVB screen and the likelihood of devices having uart
enabled. For "flashall", there is rarely a need to actually go into
userspace, because all of super is getting thrown away. We can just
flash super in the bootloader.
In the past we didn't do this because computing super.img is expensive -
both in terms of time (due to reading dependent images) and in terms of
space (it's easily over 5GB).
But we don't actually need to fully compute super.img. We can build a
sparse_file containing the metadata/headers, with additional references
to each image file containing partition data. Liblp provides the API to
do that, and here, we simply need to translate the layout to libsparse.
On Pixel, this reduces flashall time by around 35-50 seconds, or around
20% of total time, depending on whether uart is in use.
There are some caveats, in which case we'll fall back to normal
fastbootd. This does not work on non-A/B devices, on retrofit dynamic
partition devices (Pixel 3), and in some other edge-casey scenarios. If
it fails, -v will add logging information about why.
Test: fastboot flashall on Pixel 5+