Use weak assignment for SERIAL_CONSOLES in qemu configuration files

Use weak assignment for SERIAL_CONSOLES in qemu configuration files

Use weak assignment for SERIAL_CONSOLES in qemu configuration files so that
the value could serve as a default value and could be easily overridden in
configuration files like local.conf.

When using the default value for SERIAL_CONSOLES in qemux86-64,we would have
annoying messages on console complaining about respawning getty on ttyS1.
Although the value is set by purpose, at least we need to provide an easy way
to override it.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>