Bug#848911: u-boot: add support for cubox i4x4

[peter green]

Package: u-boot

The cubox i4x4 is a version of the cubox i4pro but with 4 gigabytes of 
ram (3.8 usable due to address space limitations) instead of two. It 
seems to be reasonablly popular among Debian folks. Currently Debian 
u-boot can boot the box but only leaves 2 gigabytes of ram usable.

More specifically it seems that the i4 uses four chips each four 
gigabits in size while the i4x4 also uses four chips but they are 8 
gigabits in size. Changing the ram setting is trivial but obviously 
breaks support for the cubox i4 (which is also popular among Debian folks).

Ideally we would have runtime autodetection but it doesn't seem anyone 
has stepped forward to implement/forward port it for mainline u-boot. So 
I spoke to vagrantc on irc about whether it was ok to build a seperate 
flavour for the i4x4

He seemed ok with the idea I wrote a quick patch to make it a build time 
option and build both flavours.

Interestingly the changes only seem to affect spl not u-boot itself. I'm 
not sure if it's possible to build just spl for a variant and if-so 
whether it would be a good idea.

I have tested the builds on my cubox i4x4 and they see the expected 
ammounts of ram.

Debdiff attatched.

P.S. the instructions in the u-boot readme reffer to mmcblk0 but on 
recent kernels the SD card slot on the cubox seems to come up as mmcblk1