setexpr: simplify code, improve help message

Simplify the argument checking for the "setexpr" command.  This is
done mainly to make future extensions easier.

Also improve the help message for the one argument version of the
command - this does not "load an address", but a value, which in
this context may be a plain number or a pointer dereference.

Signed-off-by: Wolfgang Denk <wd@denx.de>