~ # ls -la /tmp/a/etc/init/ | grep adb
-rw-r--r--    1 root     root          1521 Nov 12  2015 adbd-emergency-shell.conf
-rw-rw-rw-    1 root     root           254 Mar  7  2016 adbd.conf
-rw-r--r--    1 root     root           912 Sep 13  2014 android-tools-adbd.conf
-rw-r--r--    1 root     root          1003 Nov 12  2015 force-adb.conf
~ # ls -la /tmp/a/usr/bin/ | grep adb
-rwxr-xr-x    1 root     root         88604 Mar  2  2015 adbd
~ # cat /tmp/a/etc/init/adbd.conf
start on startup

pre-start script
    path="/sys/class/android_usb/android0"
    echo 0FCE >$path/idVendor || true
    echo 51A9 >$path/idProduct || true
    echo adb >$path/functions || true
    echo 1 >$path/enable || true
end script

exec /sbin/adbd