build: Add chromium prebuilt support to envsetup.sh && The core Makefile

This adds a chromium_prebuilt function to envsetup.sh that is invoked by lunch to check
whether the chromium prebuilts are up-to-date or not. If not, it will be built from source
and then the new source built version will be pulled during brunch/mka bacon to become the
new prebuilts for future builds.

This is all opt-in through the USE_PREBUILT_CHROMIUM flag. Without it being set to 1,
none of this would be ran, and regular operations will go on.

PS13:
-use export TARGET_DEVICE
-replace git -C with params compatible to old git versions

Conflicts:
	core/Makefile
	envsetup.sh

Change-Id: Ia040423e90abc2ef647717a41d9d972bd90ba780
Prebuilt chromium: Run a check for target device directory

this should fix the issue that arises if device fails and device is built again, chromium will error out because it thinks the prebuilt files exist (which they are not, directories are just created).

Signed-off-by: Ayysir <ayysir@aospal.com>
Change-Id: I595cab268e66c738ccae9900150b36d7f9626a6e
build: Fix PreBuilt Chromium refs

Change-Id: Icc553d5be1e71d8c754bc85b6906252e5690b508
Update chromium_prebuilt.sh for 5.0

Change-Id: Idfd9b28f048447bf69b0f1c710b0ee354eff829a
Fix chromium