kbuild: Extend kbuild/defconfig tags support to exuberant ctags

The following patch extends kbuild/defconfig tags support to exuberant
ctags.  The previous support is only for emacs ctags/etags programs.

This patch also corrects the kconfig regex for the emacs invocation.
Previously it would miss some instances because it assumed /^config
instead of /^[ \t]*config

Signed-off-by: Aron Griffis <aron@hp.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>