rockchip: mkimage: clarify header0 initialisation

This change set adds documentation to the header0 initialisation and
improves readability for the calculations of various offsets/lengths.

As the U-Boot SPL stage doesn't use any payload beyond what is covered
by init_size, we no longer add RK_MAX_BOOT_SIZE to init_boot_size.

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