envsetup.sh is an implementation detail of build.sh and friends and
should therefore not be sourced directly. Hence move its functionality
and just leave a warning in the original envsetup.sh along with some
documentation.

Change-Id: Ib0cfc56c7bbe4f2ce9d4cd01ea251e6982822c47
Signed-off-by: Matthias Maennich <maennich@google.com>