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>