kconfig: create links in info window

Extend the expression print helper function to allow customization of the
symbol output and use it to add links to the info window.

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>