kconfig: use long options in conf

The list of options supported by conf is growing
and their abbreviation did not resemble anything usefull.

So drop the single letter options in favour of long options.

The long options are named equal to what we know from
the make target.
The internal implmentation was changed to match this,
resulting in much more readable code.

Support for short options is dropped - no one is supposed
to call this program direct anyway.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>