panic: Make panic_timeout configurable

The panic_timeout value can be set via the command line option
'panic=x', or via /proc/sys/kernel/panic, however that is not
sufficient when the panic occurs before we are able to set up
these values. Thus, add a CONFIG_PANIC_TIMEOUT so that we can
set the desired value from the .config.

The default panic_timeout value continues to be 0 - wait
forever. Also adds set_arch_panic_timeout(new_timeout,
arch_default_timeout), which is intended to be used by arches in
arch_setup(). The idea being that the new_timeout is only set if
the user hasn't changed from the arch_default_timeout.

Signed-off-by: Jason Baron <jbaron@akamai.com>
Cc: benh@kernel.crashing.org
Cc: paulus@samba.org
Cc: ralf@linux-mips.org
Cc: mpe@ellerman.id.au
Cc: felipe.contreras@gmail.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1a1674daec27c534df409697025ac568ebcee91e.1385418410.git.jbaron@akamai.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

lib/Kconfig.debug

@@ -761,6 +761,15 @@ config PANIC_ON_OOPS_VALUE
 	default 0 if !PANIC_ON_OOPS
 	default 1 if PANIC_ON_OOPS
 
+config PANIC_TIMEOUT
+	int "panic timeout"
+	default 0
+	help
+	  Set the timeout value (in seconds) until a reboot occurs when the
+	  the kernel panics. If n = 0, then we wait forever. A timeout
+	  value n > 0 will wait n seconds before rebooting, while a timeout
+	  value n < 0 will reboot immediately.
+
 config SCHED_DEBUG
 	bool "Collect scheduler debugging info"
 	depends on DEBUG_KERNEL && PROC_FS