Makefile: Change CONFIG_SPL_PAD_TO to image offset

Change CONFIG_SPL_PAD_TO from a link address to an image offset since this is
more handy and closer to the purpose of this config.

Automatically define CONFIG_SPL_PAD_TO to CONFIG_SPL_MAX_SIZE (or 0 without
CONFIG_SPL_MAX_SIZE).

Test that CONFIG_SPL_PAD_TO >= CONFIG_SPL_MAX_SIZE if CONFIG_SPL_PAD_TO is
non-zero.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
diff --git a/Makefile b/Makefile
index 4ede937..8d0e2b8 100644
--- a/Makefile
+++ b/Makefile
@@ -486,9 +486,8 @@
 
 
 $(obj)u-boot-with-spl.bin: $(obj)spl/u-boot-spl.bin $(obj)u-boot.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
+		$(OBJCOPY) ${OBJCFLAGS} --pad-to=$(CONFIG_SPL_PAD_TO) \
+			-I binary -O binary $< $(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