update-alternatives: Drop deprecated code, update to show error message

This code has been deprecated for a while and confuses the class, lets drop it
and just give the user hard error messages instead of the current warnings/fixups.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
task.bbclass: Drop deprecated class

Any users have had time to adapt to this change by now, drop the old class.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
scripts/bitbake: Handle the case where git isn't installed cleanly

Currently the user sees ugly errors if git isn't installed, this patch
cleans up the code to correctly handle that case.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>