rockchip: mkimage: rewrite padding calculation for SD/MMC and SPI images

In (first) breaking and (then) fixing the rkspi tool, I realised that
the calculation of the required padding (for the header-size and the
2K-in-every-4K SPI layout) was not as self-explainatory as it could
have been.  This change rewrites the code (using new, common functions
in rkcommon.c) and adds verbose in-line comments to ensure that we
won't fall into the same pit in the future...

Tested on the RK3399 (with has a boot0-style payload) with SD/MMC and SPI.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>