lib: turn CONFIG_STACKTRACE into an actual option.

I was puzzled why /proc/$$/stack had disappeared, until I figured out I
had disabled the last debug option that did a 'select STACKTRACE'.  This
patch makes the option show up at config time, so it can be enabled
without enabling any of the more heavyweight debug options.
Signed-off-by: NDave Jones <davej@redhat.com>
Acked-by: NIngo Molnar <mingo@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/Kconfig.debug

@@ -1036,8 +1036,13 @@ config TRACE_IRQFLAGS
 	  either tracing or lock debugging.
 
 config STACKTRACE
-	bool
+	bool "Stack backtrace support"
 	depends on STACKTRACE_SUPPORT
+	help
+	  This option causes the kernel to create a /proc/pid/stack for
+	  every process, showing its current stack trace.
+	  It is also used by various kernel debugging features that require
+	  stack trace generation.
 
 config DEBUG_KOBJECT
 	bool "kobject debugging"