ktest: Fix config bisect with how make oldnoconfig works

With a name like 'oldnoconfig' one may think that the config generated
would disable all configs that were not defined (selecting "no" for all
options). But this is not the case. It selects the default. If a config
has a 'default y', then it is added if not specified.

This broke the config bisect, because options not specified by a config
will just use the default, where it expected to turn off. This caused an
option to be enabled that disabled an option that would break the build.
The end result was that we never found the bad config at the end of the
test.

Instead of using 'make oldnoconfig', ktest now builds the options it
expects enabled and disabled. When it turns off an option, it will no
longer remove it, but actually set it to:

 # CONFIG_FOO is not set.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>