sandbox: Adjust the order of the NO_SDL check

An option is provided to avoid using SDL in U-Boot sandbox (and drop
support for the LCD). However the check in the Makefile is too late
and warnings are printed even if NO_SDL=y is given.

Adjust the order to avoid this warning.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Jeroen Hofstee <jeroen@myspectrum.nl>