classes/populate_sdk_ext: prevent invalid TEMPLATECONF entering eSDK

classes/populate_sdk_ext: prevent invalid TEMPLATECONF entering eSDK

If you are using a repository which contains a .templateconf file that
sets TEMPLATECONF to point into a layer it contains, but you aren't
using that layer in your bblayers.conf, the eSDK would produce an error
during the preparation step of the installation. An example would be
using the poky repository but setting DISTRO to your own custom distro
and removing meta-poky from your bblayers.conf. The eSDK doesn't
support creating new build directories, so we don't care about the
templates and can thus force a known good value to prevent this from
happening.

Fixes [YOCTO #10568].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>