XO-1.75 serial console enabled by default

[Daniel Drake]

Hi,

For the next 12.1.0 build I've just enabled the XO-1.75 serial console
by default in the kernel config.
We had disabled this earlier as we had linked it to suspend/resume
crashes, but we now believe that link to have been broken.

Enabling it again now should give us some good testing before the
12.1.0 release to see that this bug has gone.

Thanks
Daniel