doc: Add section about ostree and aboot

Android Bootloader is a standard of how Android devices should implement
their bootloaders, we also use it in CentOS Automotive Stream
Distribution for some ARM boards. Here is some documentation on how
ostree works with this.

Signed-off-by: Eric Curtin <[email protected]>

docs/bootloaders.md

@@ -24,7 +24,35 @@ And that's it.
 For historical reasons, OSTree defaults to detecting the bootloader; if some GRUB files are present then OSTree will default to executing `grub2-mkconfig`.
 
 [Commented out for now, as this can lead to the system not booting in some cases.]: #
-[This can be avoided by setting `sysroot.bootloader=none` (except this should not be set on s390x).]: # 
+[This can be avoided by setting `sysroot.bootloader=none` (except this should not be set on s390x).]: #
+
+## OSTree and aboot
+
+The Android bootloader is another bootloader than may be used with ostree. It still uses the files in `/boot/loader/entries` as metadata, but the boootloader does not read these files. Android bootloaders package their kernel+initramfs+cmdline+dtb in a signed binary blob called an [Android Boot Image](https://source.android.com/docs/core/architecture/bootloader/boot-image-header). This binary blob then is written to either partition boot_a or boot_b depending on which slot is suitable.
+
+Android bootloaders by design inject kargs into the cmdline, some patches may be required in the Android bootloader implementation to ensure that the firmware does not switch between system_a and system_b partitions by populating a `root=` karg, or that a `ro` karg that is incompatible with ostree is not inserted. Conversly leaving the `androidboot.slot_suffix=` karg injecting functionality is required but this is commonplace for any Android Bootloader that does AB updates.
+
+We have two accompanying scripts that work with this type of environment:
+
+[aboot-update](https://gitlab.com/CentOS/automotive/rpms/aboot-update) is used to generate Android Boot Images to be delivered to the client.
+
+[aboot-deploy](https://gitlab.com/CentOS/automotive/rpms/aboot-deploy) reads what the current slot is according to the `androidboot.slot_suffix=` karg, writes to the alternate boot_a or boot_b slot and sets a symlink either /ostree/root.a or /ostree/root.b so that it is known which userspace directory to boot into based on the `androidboot.slot_suffix=` karg, on subsequent boots.
+
+```
++-----------------------------+                            +---------------------------------+
+|                             |    +------------------+    |                                 |
+|  firmware appends:          |    |                  |    |                                 |
+|                             +--->+ boot_a partition +--->+                                 |
+|  androidboot.slot_suffix=_a |    |                  |    |                  /ostree/root.a |
+|                             |    +------------------+    |                                 |
+|  or                         |                            | system_a partition              |
+|                             |    +------------------+    |                                 |
+|  androidboot.slot_suffix=_b |    |                  |    |                  /ostree/root.b |
+|                             +--->+ boot_b partition +--->+                                 |
+|  to cmdline                 |    |                  |    |                                 |
+|                             |    +------------------+    |                                 |
++-----------------------------+                            +---------------------------------+
+```
 
 ## GRUB and os-prober