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
(cherry picked from commit 84bbaf5a7426680b0b3723d11be852f756ed9e86)
build: update prebuilt chromium script

Prebuilt chromium: update for lollipop

Change-Id: I4e34feb26c1b4bec10fb751a10dbc52579d7cd07

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
(cherry picked from commit 32dad2952a7c9c924e18ca97336bcaefdc45fe5c)