arm: mxs: Fix mkimage invocation

Remove this ad-hoc mkimage invocation in MXS Makefile and replace
it with the standard mkimage rule instead.

This patch fixes recent build issues introduced by the patch
92a655c mkimage: Set up a file size parameter and keep it updated
These build issues could be triggered by building for example the
MX28EVK and the u-boot.sb image:

$ make mx28evk_defconfig
$ make V=1 u-boot.sb
[...]
make -f ./scripts/Makefile.build obj=arch/arm/cpu/arm926ejs/mxs u-boot.sb
  ./tools/mkimage -n arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg -T mxsimage u-boot.sb
./tools/mkimage: Can't open (null): Bad address
arch/arm/cpu/arm926ejs/mxs/Makefile:82: recipe for target 'u-boot.sb' failed
make[1]: *** [u-boot.sb] Error 1
Makefile:989: recipe for target 'u-boot.sb' failed
make: *** [u-boot.sb] Error 2

With this patch:
  ./tools/mkimage -n arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg -T mxsimage -d arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg u-boot.sb

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Juha Lumme <juha.lumme@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/cpu/arm926ejs/mxs/Makefile

@@ -74,12 +74,10 @@ u-boot.csf: u-boot.ivt u-boot.bin board/$(VENDOR)/$(BOARD)/sign/u-boot.csf
 %.sig: %.csf
 	$(call if_changed,mkcst_mxs)
 
-quiet_cmd_mkimage_mxs = MKIMAGE $@
-cmd_mkimage_mxs = $(objtree)/tools/mkimage -n $< -T mxsimage $@ \
-	$(if $(KBUILD_VERBOSE:1=), >/dev/null)
-
+MKIMAGEFLAGS_u-boot.sb = -n $< -T mxsimage
 u-boot.sb: $(src)/$(MKIMAGE_TARGET-y) u-boot.bin spl/u-boot-spl.bin FORCE
-	$(call if_changed,mkimage_mxs)
+	$(call if_changed,mkimage)
 
+MKIMAGEFLAGS_u-boot-signed.sb = -n $< -T mxsimage
 u-boot-signed.sb: $(src)/mxsimage-signed.cfg u-boot.ivt u-boot.sig spl/u-boot-spl.ivt spl/u-boot-spl.sig FORCE
-	$(call if_changed,mkimage_mxs)
+	$(call if_changed,mkimage)