gemini: create a copy-kernel for 3072k kernels

gemini: create a copy-kernel for 3072k kernels

The Raidsonic devices do not use a 2048k kernel "Kern"
partition like the Storlink reference designs. Instead
it uses a 3072k partition to fit a slightly
larger kernel.

Sadly the current OpenWrt Gemini kernel is still bigger
than 3072k so we need to make use of the Ramdisk
partition as well.

Create a special "copy-kernel" version that can deal
with the Raidsonic 3072k kernels. Tested on the
Raidsonic IB-4220-B booting kernel v6.12.66.

Fix a copy/paste error in the image generation makefile
while we are at it.

Link: https://github.com/openwrt/openwrt/pull/21686
Signed-off-by: Linus Walleij <linusw@kernel.org>