Recovery image is self-contained

Now recovery mode is self-contained, which means we don't need to mount
system.img to run shell, etc. What is needed in recovery mode is all in
the recovery ramdisk image.

Since we no longer use /system as the mount point for the system.img,
this allows us to have identical filesystem layout as the system.img.
Executables and libs are installed to /system/bin and /system/lib.
Right now, we only have adbd, sh, toybox in /system/bin but will move
static executables from /sbin to /system/bin as soon as they are
converted to dynamic executables.

system.img is mounted to /mnt/system instead.

Bug: 63673171
Test: `adb reboot recovery; adb devices` shows the device ID
Test: `adb root && adb shell` and then
$ lsof -p `pidof adbd` shows that libm.so, libc.so, etc. are loaded from
the /lib directory.

Change-Id: I801ebd18f3e0a112db3d9a11e4fbb4e49181652a
diff --git a/etc/init.rc b/etc/init.rc
index 0fc6c4c..96c37b1 100644
--- a/etc/init.rc
+++ b/etc/init.rc
@@ -22,6 +22,7 @@
     mkdir /data
     mkdir /cache
     mkdir /sideload
+    mkdir /mnt/system
     mount tmpfs tmpfs /tmp
 
     chown root shell /tmp
@@ -87,7 +88,7 @@
 service recovery /sbin/recovery
     seclabel u:r:recovery:s0
 
-service adbd /sbin/adbd --root_seclabel=u:r:su:s0 --device_banner=recovery
+service adbd /system/bin/adbd --root_seclabel=u:r:su:s0 --device_banner=recovery
     disabled
     socket adbd stream 660 system system
     seclabel u:r:adbd:s0