[PATCH] powerpc: Make early debugging options behave with oldconfig

If you undefine all the early debugging options and then run make oldconfig,
you don't get prompted to see if you want to enable any of them. This is
annoying.

AFAICT we can't do this just with a choice, because the choice is either
optional, in which case we don't get prompted, or not in which case we _must_
select early debugging.

So add a bool which controls whether we have early debugging at all, and then
if that's enabled provide the choice. The extra bool will actually be useful
in another patch I have lying around, so this is a win-win.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>