kconfig: strip 'CONFIG_' automatically in kernel configuration search

Modify the ncurses configuration tool ('make menuconfig') in a way that the
user can enter the search string (/) both with or without the leading
'CONFIG_'.

This simplifies using copy & paste from .config files because you can
select the whole word.

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>