Support shared library in recovery mode

The dyanmic linker and the linker config script for the recovery mode
(*.recovery variants) are added to PRODUCT_PACKAGES.

Also this removes the mountpoint /system_root from the recovery image as the
recovery image is self-contained and the real system image is mounted at
/mnt/system which is created automatically at runtime.

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: Ice6c75b0b81bb9be696aaff3e095d87f19278995