kbuild: adding symbols in Kconfig and defconfig to TAGS

I'm using TAGS generated from "make TAGS" to read the kernel source code.

When I met a ifdef block

	  #ifdef CONFIG_FOO
	  	 ...
	  #endif

in the soruce code I would like to know the meaning CONFIG_FOO
to decide whether I should read inside the ifdef block
or not. meaning CONFIG_FOO is well documented in Kconfig.
So it is nice if I can jump to CONFIG_FOO entry in Kconfig
from "#ifdef CONFIG_FOO" with the tag jump.

Here is the patch to add symbols in Kconfig and defconfig to TAGS
in "make TAGS" operation.

Signed-off-by: Masatake YAMATO <jet@gyve.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>