Currently the mechanism for breaking out of the dependnecy loop analysis
code is broken and doesn't work leading to bitbake appearing to hang.
Add in a custom exception for this purpose and fix the code to exit
as intended, fixing the hang and making the dependency loop code
usable again.
(Bitbake rev: e39dbd72ef44eebae32f9fe3b75a1bf789605558)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>