populate_sdk_ext.bclass: make sure OECORE_NATIVE_SYSROOT is exported.
Fixes bug 15464.
OECORE_NATIVE_SYSROOT is correctly set up and exported in the SDK's
environment file. But it's then unset in buildtools/environment-setup-*.
The value is restored in the SDK's environment file but is not exported
again.
Signed-off-by: Gauthier HADERER <ghaderer@wyplay.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>