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)

Conflicts:
	core/Makefile

Conflicts:
	core/Makefile
Fix chromium

Conflicts:
	core/Makefile
build: Clean up QCOM flag definitions

Change-Id: I66bca2db83260ccd65b82e540ee9f7961f00b030
(cherry picked from commit 269ab1996650be405d1d037da6d8f8d8110850fb)

Conflicts:
	core/qcom_target.mk
build: Add QCOM flags to Clang CFLAGS

Change-Id: I56f0d4106f5d3d27c1ace744d30c1c81f0052bbd
(cherry picked from commit 7ff848a2c0be11552d71902f414989ecfdd58c61)