[U-Boot] [WIKI] changes to "Custodian Git Trees"

[Albert ARIBAUD]

All,

I have modified http://www.denx.de/wiki/U-Boot/CustodianGitTrees to 
reflect the layered repository ad also to simplify the custodian 
operations description ; not having a local uboot branch anymore, and 
fetching repos rather than pulling them, should avoid the accidental 
merges that happen (ir)regularly.

Comment (or direct editing) welcome.

Amicalement,
-- 
Albert.