android: Implement A/B slot select.

The android_bootloader_control struct defined in bootloader_message.h
stored the A/B metadata used to decide which slot should we use to boot
the device. This pat

android: Implement A/B slot select.

The android_bootloader_control struct defined in bootloader_message.h
stored the A/B metadata used to decide which slot should we use to boot
the device. This patch implements the bootloader side of the slot
selection in a new "android_ab_select" command which decides the
current slot and updates the metadata as needed.

Bug: 32707546
Test: Booted a rpi3, updated to the other slot.
Change-Id: I9344ff5b76194160d2b466a50e84f4f423b1a98a

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