kconfig: Add search option for xconfig

Implement a simple search request for xconfig.  Currently the capabilities are
rather simple (the same as menuconfig).

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>