[Android] Handling kernel device uevent in userspace.

⚫ ueventd
    ⚬ ueventd creates and 'poll' netlink socket.
        ueventd_main [init/ueventd.c]

    ⚬ read kernel event message -> parse -> handle
        handle_device_event [init/devices.c]
            -> device_changed [init/init.c]
                -> queue_device_triggers [init/init_parser.c]
                            |
            +---------------+
            |
            v
        : search action list to know what to do for this event.
          searching action which name is device-added/removed-[device node].
          [device node] is from 'DEVPATH' variable of uevent message.

    ⚬ action list : list declared at [init_parser.c]
        • builtin action (added by 'queue_builtin_action [init/init_parser.c]')
            ex. property_init, wait_for_coldboot_done etc.
        • from script (added by 'parse_action [init/init_parser.c]')
            section "on device-added-[device path] / on device-removed-[device path]"
            ex.
                [init.rc]
                on device-added-/dev/block/mmcblk0p18
                    exec /system/bin/sh /system/bin/hello_mmc.sh

⚫ other daemons (ex usbd)
    ⚬ create and 'poll' netlink socket.