lib: turn CONFIG_STACKTRACE into an actual option.

[Dave Jones <davej@redhat.com>]

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: Dave Jones <davej@redhat.com>

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 7a638aa3545b..96c64beac12a 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1008,8 +1008,13 @@ 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"