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
8b7af4c0ee