imx: fix building for i.mx8 without spl

Building with Travis CI complained and stopped with the following error:
+cc1: fatal error: opening output file spl/u-boot-spl.cfgout: No such
file or directory
+compilation terminated.

This fixes commit caceb739ea07 ("imx: build flash.bin for i.MX8") which
took SPL being enabled on i.MX8 for granted.

Reported-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
1 file changed