build: Add chromium prebuilt support to envsetup.sh && The core Makefile [1/2]
    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

    Modifed by TeamOctOs for use under Android 5, original commit from
    https://github.com/omnirom/android_build/commit/f35d2710a9fa93376066cd6fa9a949dbd8034778#diff-bc7fc62c3359f2c1c947e75eb353a94aR1453

    Change-Id: I8855ffceccf7f6dc4c96d6c21db7b9d92a218047

Change-Id: I6b005ae881ab4c298645af8d3e74da2ae9035c72