classes/populate_sdk_base: Add setscene tasks

classes/populate_sdk_base: Add setscene tasks

do_populate_sdk was added to SSTATETASKS, but had no _setscene task
created to allow it to actually run from sstate. Add it so that SDKs can
be restored from sstate.

Note that like do_image_complete, do_populate_sdk is marked with
SSTATE_SKIP_CREATION by default so sstate is not used for them; adding
this task will allow it to work if the user overrides this default
though.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>