Use prebuilts/android/$plat/ramdisk.img

The ramdisk.img was taken from $TOP_DIR, but to fall in line with the
ARM version of the build scripts, in advance of moving to them, expect
the ramdisk.img to live in prebuilts/android/$plat/ramdisk.img, where
$plat is fvp/juno/tc2.

Change-Id: Ie225b6011409f0e34c005dba50d752784b879b1f
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>