init: document the early init boot phase

SELinux would permit the operation, or if the UIDs and GIDs resolve.

2) No checking if a service exists or has a valid SELinux domain defined

3) No checking if a service has not been previously defined in a different init script.

Early Init Boot Sequence

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The early init boot sequence is broken up into three stages: first stage init, SELinux setup, and

second stage init.

First stage init is responsible for setting up the bare minimum requirements to load the rest of the

system. Specifically this includes mounting /dev, /proc, mounting 'early mount' partitions (which

needs to include all partitions that contain system code, for example system and vendor), and moving

the system.img mount to / for devices with a ramdisk.

Note that in Android Q, system.img always contains TARGET_ROOT_OUT and always is mounted at / by the

time first stage init finishes. Android Q will also require dynamic partitions and therefore will

require using a ramdisk to boot Android. The recovery ramdisk can be used to boot to Android instead

of a dedicated ramdisk as well.

First stage init has three variations depending on the device configuration:

1) For system-as-root devices, first stage init is part of /system/bin/init and a symlink at /init

points to /system/bin/init for backwards compatibility. These devices do not need to do anything to

mount system.img, since it is by definition already mounted as the rootfs by the kernel.

2) For devices with a ramdisk, first stage init is a static executable located at /init. These

devices mount system.img as /system then perform a switch root operation to move the mount at

/system to /. The contents of the ramdisk are freed after mounting has completed.

3) For devices that use recovery as a ramdisk, first stage init it contained within the shared init

located at /init within the recovery ramdisk. These devices first switch root to

/first_stage_ramdisk to remove the recovery components from the environment, then proceed the same

as 2). Note that the decision to boot normally into Android instead of booting

into recovery mode is made if androidboot.force_normal_boot=1 is present in the

kernel commandline.

Once first stage init finishes it execs /system/bin/init with the "selinux_setup" argument. This

phase is where SELinux is optionally compiled and loaded onto the system. selinux.cpp contains more

information on the specifics of this process.

Lastly once that phase finishes, it execs /system/bin/init again with the "second_stage"

argument. At this point the main phase of init runs and continues the boot process via the init.rc

scripts.