patches/barebox: fix tftp boot sequence

Second test of $? value was not always result of command
"tftp $eth0.ipaddr/wrboot wrboot", but sometimes result of first if statement
(in case first tftp was successful).

In other words, when there was file wrboot-$eth0.ethaddr on tftp server, script
was still fetching and executing wrboot.

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>