ramips: Fix EX400 imagebuilder

ramips: Fix EX400 imagebuilder

A factory image for DNA EX400 depends on an initramfs image and they
were explicitly removed from the imagebuilder recently. Now the factory
image creation fails miserably and it also affects custom image creation
with the firmware selector.

Add the initramfs kernel to the staging so that it's shipped with the
imagebuilder. Also remove a image build target added solely for DNA EX400.

Tested by creating a factory and syspupgrade images locally with
the imagebuilder and verified their functionality.

Related work
  c85348d9abf4 ("imagebuilder: remove initramfs image files")

Fixes: fea2264d9fdd ("ramips: mt7621: Add DNA Valokuitu Plus EX400")
Signed-off-by: Mauri Sandberg <maukka@ext.kapsi.fi>
---
v4: use append-image-stage, remove Build/kernel-initramfs-bin
v3: adjust commit subject
v2: remove fix for inconsistent line ending elsewhere in the file
Link: https://github.com/openwrt/openwrt/pull/16659
Signed-off-by: Robert Marko <robimarko@gmail.com>