[POWERPC] print backtrace when entering xmon

xmon does not print a backtrace per default. This is bad on systems with
USB keyboard, the most needed info about the crash is lost.
print a backtrace during the very first xmon entry.

Booting with xmon=nobt disables the autobacktrace functionality.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
index d57930d..465e743 100644
--- a/arch/powerpc/kernel/setup-common.c
+++ b/arch/powerpc/kernel/setup-common.c
@@ -441,6 +441,8 @@
 
 int __initdata do_early_xmon;
 #ifdef CONFIG_XMON
+extern int xmon_no_auto_backtrace;
+
 static int __init early_xmon(char *p)
 {
 	/* ensure xmon is enabled */
@@ -449,6 +451,8 @@
 			xmon_init(1);
 		if (strncmp(p, "off", 3) == 0)
 			xmon_init(0);
+		if (strncmp(p, "nobt", 4) == 0)
+			xmon_no_auto_backtrace = 1;
 		if (strncmp(p, "early", 5) != 0)
 			return 0;
 	}