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

(vendor repository for script)

Change-Id: I40f8c8dbd2a8a84a5c1b9f47ee04180a71ef4e07

Conflicts:
	core/Makefile
	envsetup.sh
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