doc: android: Add simple guide for A/B updates

Add a short documentation for A/B enablement and 'ab_select' command
usage.

Signed-off-by: Ruslan Trofymenko <ruslan.trofymenko@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Alistair Strachan <astrachan@google.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

diff --git a/doc/android/ab.txt b/doc/android/ab.txt
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+Android A/B updates
+===================
+
+Overview
+--------
+
+A/B system updates ensures modern approach for system update. This feature
+allows one to use two sets (or more) of partitions referred to as slots
+(normally slot A and slot B). The system runs from the current slot while the
+partitions in the unused slot can be updated [1].
+
+A/B enablement
+--------------
+
+The A/B updates support can be activated by specifying next options in
+your board configuration file:
+
+    CONFIG_ANDROID_AB=y
+    CONFIG_CMD_AB_SELECT=y
+
+The disk space on target device must be partitioned in a way so that each
+partition which needs to be updated has two or more instances. The name of
+each instance must be formed by adding suffixes: _a, _b, _c, etc.
+For example: boot_a, boot_b, system_a, system_b, vendor_a, vendor_b.
+
+As a result you can use 'ab_select' command to ensure A/B boot process in your
+boot script. This command analyzes and processes A/B metadata stored on a
+special partition (e.g. "misc") and determines which slot should be used for
+booting up.
+
+Command usage
+-------------
+
+    ab_select <slot_var_name> <interface> <dev[:part_number|#part_name]>
+
+for example:
+
+    => ab_select slot_name mmc 1:4
+
+or
+
+    => ab_select slot_name mmc 1#misc
+
+Result:
+
+    => printenv slot_name
+    slot_name=a
+
+Based on this slot information, the current boot partition should be defined,
+and next kernel command line parameters should be generated:
+
+ - androidboot.slot_suffix=
+ - root=
+
+For example:
+
+    androidboot.slot_suffix=_a root=/dev/mmcblk1p12
+
+A/B metadata is organized according to AOSP reference [2]. On the first system
+start with A/B enabled, when 'misc' partition doesn't contain required data,
+the default A/B metadata will be created and written to 'misc' partition.
+
+References
+----------
+
+[1] https://source.android.com/devices/tech/ota/ab
+[2] bootable/recovery/bootloader_message/include/bootloader_message/bootloader_message.h