vendor/omni: Add chromium_prebuilt.sh

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

Change-Id: I3141defcbd4108178881f7203f3105aecc27378b
Merge "vendor/omni: Add chromium_prebuilt.sh" into android-4.4
vendor/omni: Fix-ups for prebuilt Chromium

- Make copying far more reliable, even on non-clean builds
- Use $OUT in script in all cases, apparently get_build_var OUT_DIR isn't reliable.
envsetup.sh, please -_-

Change-Id: I27fa9bc7a043c32bcb29e7667e86b40226222a5a
Merge pull request #9 from omnirom/android-4.4

kjl