Clearing sbss/bss at bootup

[Justin Hibbits]

Been looking at bootup issues on the e500, and many of the issues I
see are caused by static variables not being initialized to 0 early
enough.  The code doesn't bzero() the bss/sbss until pretty late in
bootup.  Is there a reason why that is?  Why can't the bss be cleared
immediately?  Do loaders pass in data at addresses that may overlap
the bss?

- Justin