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/include/configs/MPC8313ERDB.h b/include/configs/MPC8313ERDB.h
index 275d4f2..c28dfe0 100644
--- a/include/configs/MPC8313ERDB.h
+++ b/include/configs/MPC8313ERDB.h
@@ -52,7 +52,7 @@
 #define CONFIG_SYS_TEXT_BASE	0x00100000 /* CONFIG_SYS_NAND_U_BOOT_DST */
 #define CONFIG_SYS_TEXT_BASE_SPL 0xfff00000
 #define CONFIG_SPL_MAX_SIZE	(4 * 1024)
-#define CONFIG_SPL_PAD_TO	0xfff04000
+#define CONFIG_SPL_PAD_TO	0x4000
 
 #define CONFIG_SYS_NAND_U_BOOT_SIZE  (512 << 10)
 #define CONFIG_SYS_NAND_U_BOOT_DST   0x00100000