toaster: customrecipe Only show download icon or button if it's possible

toaster: customrecipe Only show download icon or button if it's possible

If the based on recipe has not yet been checked out/cloned we cannot
generate the custom image recipe file that uses it. So disable/remove
the option to download it.

[YOCTO #9425]

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