uboot-imx

travis.yml: fix 'set +e' in build script

The build script should not manipulate shell flags (especially '-e').
A non-zero exit value can also be catched with 'cmd || ret=$?'.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>