rockchip: mkimage: force 2KB alignment for init_size

The Rockchip BootROM relies on init_size being aligned to 2KB
(see https://lists.denx.de/pipermail/u-boot/2017-May/293268.html).

This pads the image to 2KB both for SD card images and SPI images
and uses a common symbolic constant for the alignment.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/rkcommon.h b/tools/rkcommon.h
index 75a4ed6..8790f1c 100644
--- a/tools/rkcommon.h
+++ b/tools/rkcommon.h
@@ -10,6 +10,7 @@
 
 enum {
 	RK_BLK_SIZE		= 512,
+	RK_INIT_SIZE_ALIGN      = 2048,
 	RK_INIT_OFFSET		= 4,
 	RK_MAX_BOOT_SIZE	= 512 << 10,
 	RK_SPL_HDR_START	= RK_INIT_OFFSET * RK_BLK_SIZE,