tracing/fastboot: Document the need of initcall_debug

To use boot tracer, one should pass initcall_debug as well as
ftrace=initcall to the command line.
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4A48735E.9050002@cn.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/Kconfig

@@ -226,13 +226,13 @@ config BOOT_TRACER
 	  the timings of the initcalls and traces key events and the identity
 	  of tasks that can cause boot delays, such as context-switches.
 
-	  Its aim is to be parsed by the /scripts/bootgraph.pl tool to
+	  Its aim is to be parsed by the scripts/bootgraph.pl tool to
 	  produce pretty graphics about boot inefficiencies, giving a visual
 	  representation of the delays during initcalls - but the raw
 	  /debug/tracing/trace text output is readable too.
 
-	  You must pass in ftrace=initcall to the kernel command line
-	  to enable this on bootup.
+	  You must pass in initcall_debug and ftrace=initcall to the kernel
+	  command line to enable this on bootup.
 
 config TRACE_BRANCH_PROFILING
 	bool