This patch reintroduce the ALLSOURCE_ARCHS support for tags/TAGS/
cscope targets. The Kbuild previously has this feature, but after
moving the targets into scripts/tags.sh, ALLSOURCE_ARCHS disappears.

It's something like this:

	$ make ALLSOURCE_ARCHS="x86 mips arm" tags cscope
Signed-off-by: NJike Song <albcamus@gmail.com>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

Add kbuild.txt to Documentation/kbuild
More stuff can be added later - at least we have
som of the varous environment variables documented now.
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>