Previously vexpress would use setenv to define tftp commands, then setup
the dispatcher ip, common uboot commands, and then boot, which triggered
tftp commands.

Now that tftp commands are used directly prior to the boot command,
ensure that the correct network settings are in place prior to any tftp
commands are run.

Change-Id: I6b9a4622792060cacf0d7c3b65cce10801c3b369
Signed-off-by: Dean Birch <dean.birch@arm.com>