[PATCH] common/cmd_elf.c: Enable loadaddr as parameter in bootvx command

In the bootvx command the load address was only read from the env
variable "loadaddr" and not optionally passed as paramter as described
in the help. This is fixed with this patch. The behaviour is now the
same as in the bootelf command.

Signed-off-by: Stefan Roese <sr@denx.de>