rockchip: mkimage: set init_boot_size to avoid confusing the boot ROM

This change restores the earlier setting of init_boot_size to include
the maximum area covered by the the boot ROM of each chip for resolve
issues with back-to-bootrom functionality reported by Kever and Heiko.

To ensure that we don't run into the same issue again in the future,
I have updated the comments accordingly and added a reference to the
mailing list archive (there's some very helpful info from Andy Yan
that provides background on the BootROM requirements regarding these
fields).

See https://lists.denx.de/pipermail/u-boot/2017-May/293267.html

 for
some background (by Andy Yan) of how the BootROM processes this field.

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