Recovery partition has same layout as system

Recovery partition has same layout as system. In other words,
executables are installed to <recovery_root>/system/bin and libs are
installed to ../system/lib.

This is made possible because the recovery partition is now
self-contained, i.e., shell, adbd, etc. are hosted in the recovery
partition and thus we no longer has to mount the real system.img
to /system. So /system is now available to executables built for
recovery mode.

Bug: 63673171
Test: `adb reboot recovery; adb devices` shows the device ID
Test: Select 'mount /system' in the recovery mode, then `adb shell`.
$ lsof -p `pidof adbd` shows that libm.so, libc.so, etc. are loaded from
the /lib directory.

Change-Id: I6c4f903ad21fd3dd10a07c4588be85d3d678c099