Add chromium_prebuilt.sh

(Also includes: omnirom/android_vendor_omni@9a7b0bf)

This script will pull the necessary libs from a complete build and generate proper
makefiles in order to support using a prebuilt Chromium. Files are placed in
prebuilts/chromium/-devicenamehere-, and are not tracked by git, so they are completely
out of the way.

PS3:
-replace git -C with params compatible to old git versions

Original commit by AOSPA-legacy .
Credits to Jiangyi

Signed-off-by: k2wl <kar2wlan@gmail.com>
Added build script and user variant for official devices

Signed-off-by: k2wl <kar2wlan@gmail.com>