[U-Boot] [PATCH] arm, imx: fix spl compile for mxs boards

[Craig Lilley]

> 
> ...I do not understand why I can compile clean (u-boot-imx):
> 
> ./tools/buildman/buildman mx28evk
> boards.cfg is up to date. Nothing to do.
> Building current source for 4 boards (4 threads, 2 jobs per thread)
>     4    0    0 /4      0:00:48  : mx28evk_auart_console
> 
> And all mxb boards are compiled clean, too.
> 

It appears you can compile clean because that defconfig 
(mx28evk_auart_console) doesnt fully enable the serial output.

You still need to define CONFIG_SPL_SERIAL_SUPPORT somewhere or else 
mxs_spl_console_init() just optimises away.

When I define this in configs/mxs.h I get the build error.

Can you re-consider applying this patch given the above?