arm: mx6: specify SPL padding

Specify standard padding for payload to 68KB. This is derived from
the maximum header size plus maximum SPL size. It matches the
already defined offset for SD/eMMC devices (69KB) too. This allows
to use the u-boot-with-spl.imx build target to generate a directly
flashable image which can be flashed using:

  dd if=u-boot-with-spl.imx of=/dev/mmcblk0 bs=512 skip=2

While the patch has been created with SD/eMMC in mind, this also
works with other boot media. The board file needs to configure the
media specific (absolute) payload offset accordingly. Especially
the IVT offset is boot media specific and can be retrieved from the
reference manual (Table 8-25. Image Vector Table Offset and Initial
Load Region Size). For NAND boot a define like this should do the
job:

 #define CONFIG_SYS_NAND_U_BOOT_OFFS (SPL_PAD_TO + 0x400)

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>

include/configs/imx6_spl.h

@@ -29,6 +29,12 @@
 #define CONFIG_SPL_TEXT_BASE		0x00908000
 #define CONFIG_SPL_MAX_SIZE		0x10000
 #define CONFIG_SPL_STACK		0x0091FFB8
+/*
+ * Pad SPL to 68KB (4KB header + 64KB max size). This allows to write the
+ * SPL/U-Boot combination generated with u-boot-with-spl.imx directly to a
+ * boot media (given that boot media specific offset is configured properly).
+ */
+#define CONFIG_SPL_PAD_TO		0x11000
 
 /* NAND support */
 #if defined(CONFIG_SPL_NAND_SUPPORT)