[Arm-netbook] u-boot A20 sunxi, added SPL dual MMC boot

[Luke Kenneth Casson Leighton]

http://git.rhombus-tech.net/?p=u-boot.git;a=shortlog;h=refs/heads/sunxi

it's a feature that's present in quite a lot of versions of u-boot but
not in the sunxi one: i added a means to check *two* MMC slots in the
early SPL loader.

the problem without such a feature is: you can only boot from one MMC
slot, even if you compile u-boot with support for two MMC slots...
because the early SPL loader *only recognises loading u-boot from one
MMC slot*.

for those people not familiar with the A20 boot process:
http://linux-sunxi.org/BROM#A10.2FA20

the BROM loads from a very specific sector of either MMC0 or MMC2 (in
that order).  maximum code size: 16k bytes.  loading a full u-boot is
not possible so u-boot has to be boot-strapped (aka SPL loader).  the
*existing* sunxi SPL loader *only* supports loading from a *single*
MMC card slot.

l.

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