ranchu: Add ARM64-specific root/ramdisk config files.

Emulating ARM64 devices requires using binaries built from qemu-android [1]
instead of the regular Android emulator.

qemu-android is based on a more recent fork of upstream QEMU, and implements
a new virtual board, named 'ranchu', that differs from 'goldfish' in many
ways.

These files are to be referenced from $AOSP/build/target/board/generic_arm64/
They come from device/linaro/ranchu, but are copied here because generic_arm64
cannot reference vendor-specific files.

Note that the corresponding system images still use most of the 'goldfish'
specific guest libraries in this directory though, which is why it's easier
to place them here.

BUG=17154406

Change-Id: I641efb9adb0454f70e74e74cb6f75594f4da6a8c
Merge "ranchu: Add ARM64-specific root/ramdisk config files."