Kconfig: consolidate the "Kernel hacking" menu

Move the source of lib/Kconfig.debug and arch/$(ARCH)/Kconfig.debug to
the top-level Kconfig.  For two architectures that means moving their
arch-specific symbols in that menu into a new arch Kconfig.debug file,
and for a few more creating a dummy file so that we can include it
unconditionally.

Also move the actual 'Kernel hacking' menu to lib/Kconfig.debug, where
it belongs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Kconfig b/Kconfig
index 1a5c0a5..a5997d6 100644
--- a/Kconfig
+++ b/Kconfig
@@ -30,3 +30,5 @@
 source "crypto/Kconfig"
 
 source "lib/Kconfig"
+
+source "lib/Kconfig.debug"