default-distrovars: Have KERNEL_CONSOLE reference SERIAL_CONSOLES

default-distrovars: Have KERNEL_CONSOLE reference SERIAL_CONSOLES

Currently, KERNEL_CONSOLE has a default value of "ttyS0".  However, Arm
machines and those using virtio serial prefer to use "ttyAMA0" or "hvc0"
(or something else).  These are usually defined by the machine config
file as SERIAL_CONSOLES, which has one or more entries.  Take the first
one of those instead of ttyS0, but default back to ttyS0 if nothing is
set.

Also, use this variable in the efi wic file instead of "ttyS0".
Of note, this changes the default speed of the default kernel console
from undefined (9600) to 115200.  This allows for users of the
mkefidisk.wks to work as before but any users of this variable could see
changed behavior and would now need to define this as:
KERNEL_CONSOLE ?= "ttyS0,9600"

This includes revisions suggested by Quentin Schulz and Ross Burton.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>