Bug#864368: Expand kernel serial port configuration

[Dave Hansen <dave.hansen@intel.com>]

On 06/07/2017 01:14 PM, Ben Hutchings wrote:
> On Wed, 2017-06-07 at 18:10 +0100, Ben Hutchings wrote:
>>> 	CONFIG_SERIAL_8250_RUNTIME_UARTS=16
>> That seems reasonable.
> [...]
> 
> Actually, having looked further at what this does, I'm unconvinced
> about changing it.  It can always be overridden through the
> '8250.nr_uarts' kernel parameter where needed.

On my system, this meant that I could not use the serial console to do
an install.  Doing console=ttyS4 on the command-line didn't work because
the kernel didn't support anything after ttyS3.

I guess we could also teach folks to add nr_uarts=8 (or whatever) to
their kernel command-line, which also works around this.  But, it would
be handy to have it work by default.

I don't think there's really any overhead to raising 'nr_uarts', either.
 It makes some (unimportant) loops longer, but I don't think there's any
real downsides.

FWIW, a lot of distros make it pretty large.  My Ubuntu desktop has it
set to 48 and Fedora evidently sets it to 32.