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/arch/hexagon/Kconfig.debug b/arch/hexagon/Kconfig.debug
new file mode 100644
index 0000000..22a162cd
--- /dev/null
+++ b/arch/hexagon/Kconfig.debug
@@ -0,0 +1 @@
+# dummy file, do not delete