meta-facebook: simplify SERIAL_CONSOLES

The majority of our hardware uses ttyS4 and 57600.  Rather than have
each machine specify this, put it into a generic "facebook" override
as the default for all our machines.  Assuming OVERRIDES is set
correctly, machines can still make specific overrides as alternatives.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Id8b704163dc57d94e27481dba7aef1bdf66aeee5
diff --git a/meta-facebook/conf/machine/include/facebook.inc b/meta-facebook/conf/machine/include/facebook.inc
index 4951a60..47b641d 100644
--- a/meta-facebook/conf/machine/include/facebook.inc
+++ b/meta-facebook/conf/machine/include/facebook.inc
@@ -16,3 +16,5 @@
 
 IMAGE_FEATURES:remove = "obmc-ikvm"
 
+# Most of our hardware uses the default ttyS4 but at 57600.
+SERIAL_CONSOLES:facebook = "57600;ttyS4"