[U-Boot] Set up stdio earlier when using driver model <--- breaks sbc8548 booting.

[Paul Gortmaker <paul.gortmaker@windriver.com>]

Testing latest master on sbc8548 (ppc e500v2 single core) and it hangs
at the "Net: " line; a working boot shows the full "Net: " line as:

 -------------
PCI: Host, 64 bit, 66 MHz, sync, arbiter
  00:01.0     - 8086:1026 - Network controller
PCI1: Bus 00 - 00

PCIe1: disabled
In:    serial
Out:   serial
Err:   serial
Net:   eTSEC0 [PRIME], eTSEC1
Hit any key to stop autoboot:  0
 -------------

So we never see the eTSEC0 or any other output after "Net: ".

My 1st bisect led to my own commit:

 -------------
commit 2bf4207b8a452476a591d733c6b8f09b337acc08
Author:     Paul Gortmaker <paul.gortmaker@windriver.com>
AuthorDate: Thu Aug 14 10:42:52 2014 -0400
Commit:     York Sun <yorksun@freescale.com>
CommitDate: Fri Nov 14 11:12:13 2014 -0800

    sbc8548: enable and test CONFIG_SYS_GENERIC_BOARD
 -------------

...but that is a red herring, since I'd tested it on master at Aug14,
but it wasn't committed to master until three months later.  So the
breakage is in that 3 month window.

Since I recorded the original baseline I'd tested on, I restarted the
bisect with that baseline as good and the above 2bf42 as bad, and just
added the oneline change for CONFIG_SYS_GENERIC_BOARD manually at each
bisect point.  Doing that led me unequivocally to:

 -------------
  commit 294b91a5817147d4b7f47be2ac69bac2a1f26491
  Author: Simon Glass <sjg@chromium.org>
  Date:   Wed Sep 3 17:37:00 2014 -0600

    Set up stdio earlier when using driver model
 -------------
    
Based on a part of that commit log, it says "Should there be any
problems with this approach they can be dealt with as boards are
converted over to use driver model for serial."  So maybe the sbc8548 is
just missing some additional conversion?  Oddly it seems it is dying at
network device probing and not in/out/err that use serial as stdio.

Any hints on what to look at next to solve this would be appreciated. I
had a look at this link:

http://www.denx.de/wiki/U-Boot/DriverModel

..but wasn't sure where to go from there, since I'm still unsure what
the real root of the breakage is.

Thanks,
Paul.