net/bootp.c: fix tftp load if autoload environment var isn't set

net/bootp.c: fix tftp load if autoload environment var isn't set

Commit 093498669 (Put common autoload code into auto_load() function)
broke handling of autoload environment variable not being set.
The bootp/dhcp code will just keep on requesting IP address forever
and never start TFTP download.

Fix it by moving TftpStart() outside the conditional like it was before.

Signed-off-by: Peter Korsgaard <[email protected]>
Tested-by: Fabio Estevam <[email protected]>
Acked-by: Simon Glass <[email protected]>