klogd: make it work on non-linux systems

The klogctl() interface allows changing the console loglevel, but is
Linux-specific. The more portable method of reading from _PATH_KLOG is
added as an alternative.

Adapted from the Debian kFreeBSD patch at:
http://svn.debian.org/viewsvn/d-i/people/slackydeb/kfreebsd/busybox/1.14/debian/klogd.diff

Signed-off-by: Jeremie Koenig <jk@jk.fr.eu.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

sysklogd/Config.src

@@ -109,7 +109,6 @@ config FEATURE_LOGREAD_REDUCED_LOCKING
 config KLOGD
 	bool "klogd"
 	default y
-	depends on PLATFORM_LINUX
 	help
 	  klogd is a utility which intercepts and logs all
 	  messages from the Linux kernel and sends the messages
@@ -117,6 +116,22 @@ config KLOGD
 	  you wish to record the messages produced by the kernel,
 	  you should enable this option.
 
+config FEATURE_KLOGD_KLOGCTL
+	bool "Use the klogctl() interface"
+	default y
+	depends on KLOGD && PLATFORM_LINUX
+	help
+	  The klogd applet supports two interfaces for reading
+	  kernel messages. Linux provides the klogctl() interface
+	  which allows reading messages from the kernel ring buffer
+	  independently from the file system.
+
+	  If you answer 'N' here, klogd will use the more portable
+	  approach of reading them from /proc or a device node.
+	  However, this method requires the file to be available.
+
+	  If in doubt, say 'Y'.
+
 config LOGGER
 	bool "logger"
 	default y