Maximum MSGBUFSIZE increased

Hi,

I've committed a change that allows the maximum MSGBUFSIZE to be increased
to 248KB.  Previously the maximum was 56KB (and it was easy to overrun that
with USB debugging turned on).

Note, that this change alters the base address for the message buffer, so
when you boot with a new kernel, `dmesg` won't show any content from a
kernel built before this change.

Thanks,

J

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