bitbake: toaster: custom image enable layer add, protect pre-cloned layers

bitbake: toaster: custom image enable layer add, protect pre-cloned layers

When creating custom image recipes, the layer add for new layers
needs missing xhrLayerUrl data. Also, code is needed to check
and inform user if the newly added layer has not been cloned yet,
and provide helpful error message instead of the current frozen
dialog.

[YOCTO #12887]

(Bitbake rev: b310031972a53d0881a87a627f07bdcf7d9c6b79)

Signed-off-by: David Reyna <David.Reyna@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>