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 caceb739 ("imx: build flash.bin for i.MX8") which
took SPL being enabled on i.MX8 for granted.
Reported-by: NStefano Babic <sbabic@denx.de>
Signed-off-by: NMarcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: NPeng Fan <peng.fan@nxp.com>

arch/arm/mach-imx/Makefile

@@ -92,7 +92,9 @@ IMX_CONFIG = $(CONFIG_IMX_CONFIG:"%"=%)
 ifeq ($(CONFIG_ARCH_IMX8), y)
 CNTR_DEPFILES := $(srctree)/tools/imx_cntr_image.sh
 IMAGE_TYPE := imx8image
+ifeq ($(CONFIG_SPL_BUILD),y)
 SPL_DEPFILE_EXISTS := $(shell $(CPP) $(cpp_flags) -x c -o spl/u-boot-spl.cfgout $(srctree)/$(IMX_CONFIG); if [ -f spl/u-boot-spl.cfgout ]; then $(CNTR_DEPFILES) spl/u-boot-spl.cfgout; echo $$?; fi)
+endif
 DEPFILE_EXISTS := $(shell $(CPP) $(cpp_flags) -x c -o u-boot-dtb.cfgout $(srctree)/$(IMX_CONFIG); if [ -f u-boot-dtb.cfgout ]; then $(CNTR_DEPFILES) u-boot-dtb.cfgout; echo $$?; fi)
 else ifeq ($(CONFIG_ARCH_IMX8M), y)
 IMAGE_TYPE := imx8mimage