scripts/oe-buildenv-internal: Ensure we detect the SDK/ADT and error out

scripts/oe-buildenv-internal: Ensure we detect the SDK/ADT and error out

The SDK/ADT may ship with a python installed which may not have all the modules
need for a bitbake build. We should therefore detect if its already present in the
environment and error out in this case, asking the user to use a clean environment.

This also removes the potential for any other conflict between the two.

[YOCTO #2979]

(From OE-Core rev: 9496d4cd77ae632251b4262b63be857fc4fcb31e)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>