bcm63xx-cfe: install into image staging dir

bcm63xx-cfe: install into image staging dir

Currently, bcm63xx-cfe is being installed into kernel build dir, however
that does not work for Image Builder as only certain artifacts from kernel
build dir are included in Image Builder.

So, simply install bcm63xx-cfe into image staging dir so its artifacts can
be used in Image Builder as well.

Fixes: #18408
Fixes: #18409
Link: https://github.com/openwrt/openwrt/pull/18463
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 4a7de50769101962fd9ecc15465f6a11f0e81f9f)