Flashing A/B android builds

[g14]

I'm trying to flash an AB aosp build to my vim3 board. The official docs only describe how to flash `no_ab` builds (https://source.android.com/docs/setup/create/devices#vim3_and_vim3l_boards).This is how I'm trying to flash A/B images on my board.

* Compiled the OS with `TARGET_USE_AB_SLOT=true`

* Flashed the `u-boot_kvim3l_ab.bin` instead of `noab` firmware and formatted in emmc.

* This resulted in the following partitions being created on my board:

https://drive.google.com/file/d/1KRZ4qEy8CyzL4UPuJHsLs4zmG2t4DRKh/view?usp=sharing

At this point, if I run the `fastboot flashall` command it errors out with a message saying `failed to identify current slot`. `fastboot set_active a` fails as well with the error `Device does not support slots`.

I've tried flashing the partitions manually as described in the above link, while flashing dtbo and boot images to both A and B versions, but the boot fails at the `applying dtbo` step.