changes to "Custodian Git Trees"

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.