spl: Change PAD_TO to CONFIG_SPL_PAD_TO

This was already used by some SPL targets, and allows the pad amount to
be specified by board config headers rather than only in makefile
fragments.

Also supply a pad-to of zero if the variable is undefined.  It works
without this, but this avoids relying on undocumented behavior.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
diff --git a/Makefile b/Makefile
index 9d43499..7e16da9 100644
--- a/Makefile
+++ b/Makefile
@@ -485,8 +485,12 @@
 $(obj)u-boot.dis:	$(obj)u-boot
 		$(OBJDUMP) -d $< > $@
 
+
+
 $(obj)u-boot-with-spl.bin: $(obj)spl/u-boot-spl.bin $(obj)u-boot.bin
-		$(OBJCOPY) ${OBJCFLAGS} --pad-to=$(PAD_TO) -O binary $(obj)spl/u-boot-spl $(obj)spl/u-boot-spl-pad.bin
+		$(OBJCOPY) ${OBJCFLAGS} --pad-to=$(or $(CONFIG_SPL_PAD_TO),0) \
+			-O binary $(obj)spl/u-boot-spl \
+			$(obj)spl/u-boot-spl-pad.bin
 		cat $(obj)spl/u-boot-spl-pad.bin $(obj)u-boot.bin > $@
 		rm $(obj)spl/u-boot-spl-pad.bin