arm64: Fix aosp_arm64 system image.

This patch ensures that the system image produced for the 'aosp_arm64'
build products can actually run under emulation with the new qemu-android
binaries [1]

The main issue is that the virtual board is named 'ranchu' instead of
'goldfish' (because the virtual hardware is _very_ different), and thus
requires specific files for the boot to start properly

(in particular fstab.ranchu is required to mount the system, data and
cache partitions, otherwise init will fail badly because /system/bin/
and /data/ are missing important files).

IMPORTANT: This requires the files under device/generic/goldfish/ from:

    https://android-review.googlesource.com/#/c/105020/

[1] Binaries built from https://qemu-android.googlesource.com/qemu-android

BUG=17154406

Change-Id: Ic40360bf56e32aab708551c810000467d23793d4