toaster: prevent infinite loop when finding task dependencies

toaster: prevent infinite loop when finding task dependencies

Toaster occasionally records a task which depends on itself.
This causes a problem when trying to display that task if it
is "covered" by itself, as the code does the following: for
task A, find a task B which covers A; then, recursively
find the task which covers B etc. If B == A, this loop becomes
infinite and never terminates.

To prevent this, add the condition that, when finding a task B
which covers A, don't allow B == A.

[YOCTO #9952]

Signed-off-by: Elliot Smith <elliot.smith@intel.com>