toaster: disable/enable "Add layer" button according to input's content

toaster: disable/enable "Add layer" button according to input's content

In the import layer page, the "Add layer" button in the layer dependencies
section doesn't accurately reflect whether the layer name in the
corresponding input can be added. A partial or empty layer name can
leave the button active, such that when it is clicked, a
previously-selected layer can be accidentally added.

Fix by keeping track of the items currently available in the typeahead,
only activating the "Add layer" button when the input matches the name
of one of those items.

[YOCTO #8511]

Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>