cmd_setexpr: allow memory addresses in expressions

This patch add functionality to use memory addresses in expressions.
This increases the power of expressions substantially

It adheres to the standard convemtions: memory addresses can be given
in the format *address (e.g. *1000)

Rationale for this change is that it allows masking off bits from a
byte that is obtained by reading data from e.g. i2c.

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>

Fix warning: control reaches end of non-void function
Signed-off-by: Wolfgang Denk <wd@denx.de>