classes/populate_sdk_ext: detect and warn if running in OE environment

classes/populate_sdk_ext: detect and warn if running in OE environment

If you run the extensible SDK environment setup script in a shell
session where oe-init-build-env has been run already, and attempt to use
the two together, strange things happen - you may not even be running
devtool from the extensible SDK, but the OE tree. This isn't a supported
use case anyway, so show a warning recommending starting a new shell
session.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>