tty: Add handler for local tty mirroring

This change adds a simple handler for mirroring data to local tty
device, like a hardware UART.

This is specified through a new config parameter 'local-tty'.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/obmc-console.conf.sample b/obmc-console.conf.sample
index a5b8808..d473af2 100644
--- a/obmc-console.conf.sample
+++ b/obmc-console.conf.sample
@@ -4,3 +4,6 @@
 # For VUART devices, we can specify the LPC address and SIRQ parameters
 lpc-address = 0x3f8
 sirq = 4
+
+# To mirror to a local tty device (typically a hardware UART), set local-tty
+# local-tty = ttyS0