configs: Rename environment variable fit_loadaddr to addr_fit

configs: Rename environment variable fit_loadaddr to addr_fit

This is the first part of a larger effort I would like to propose to
unify and simplify the default set of environment variables.

When many early environment variables were named there were fewer images
being loaded, usually just a kernel. At this time names like 'loadaddr'
would suffice. Now we have more images and many more commands that act on
them, often re-using the same variable for several different uses. The
contents of a variable are also not immediately known causing one to have
to look up a chain of variables to understand what a command is actually
doing. I suggest the following.

To start, all variables containing names should be prefixed with name_
and addresses with addr_. This is like how K2 already does things and
allows for simple universal commands like:

get_fdt_nfs=nfs ${addr_fdt} /boot/${name_fdt}

Which is very clear on what is intended here and would work across all
board that using the this naming convention.

We can do this one variable at a time, start here with addr_fit.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Andreas Dannenberg <dannenberg@ti.com>