add printk.time=1 boot-time hint to Kconfig.debug help text

Cite Documentation/kernel-parameters.txt for an alternative to
building with PRINTK_TIME compiled in.
Signed-off-by: NJim Cromie <jim.cromie@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

lib/Kconfig.debug

@@ -7,7 +7,8 @@ config PRINTK_TIME
 	  included in printk output.  This allows you to measure
 	  the interval between kernel operations, including bootup
 	  operations.  This is useful for identifying long delays
-	  in kernel startup.
+	  in kernel startup.  Or add printk.time=1 at boot-time.
+	  See Documentation/kernel-parameters.txt
 
 config ENABLE_WARN_DEPRECATED
 	bool "Enable __deprecated logic"