kbuild: script to check for undefined Kconfig symbols

To avoid having to look manually for used but undefined Kconfig variables,
I've written a script which tries do this efficiently, in case all other
attention fail.  It accounts for _MODULE suffix and for UML_ prefixes to
Kconfig variable, but otherwise looks for exact matches (i.e.  \<CONFIG_;
this is done to exclude macros like MMCONFIG_).

Undefined Kconfig variables should be not be removed without care, but for
instance arch/i386/boot/ uses a bunch of undefined Kconfig vars:

$ scripts/checkunknowndefines.sh arch/i386/boot/
arch/i386/boot/video.h uses undefined symbol VIDEO_400_HACK
arch/i386/boot/video-vga.c uses undefined symbol VIDEO_400_HACK
arch/i386/boot/video.c uses undefined symbol VIDEO_RETAIN
arch/i386/boot/video.h uses undefined symbol VIDEO_RETAIN
arch/i386/boot/video.h uses undefined symbol VIDEO_SVGA
arch/i386/boot/video.h uses undefined symbol VIDEO_VESA
arch/i386/boot/video-vesa.c uses undefined symbol VIDEO_VESA

It should also be ...