firmware: Mark Intel/Lantiq firmware packages as nonshared

firmware: Mark Intel/Lantiq firmware packages as nonshared

Package the firmware files in the target specific build step and not in
the architecture common step. The architecture common step is not
necessary build for the ipq40xx target. If it is build for a different
target these packages are not packaged at all. This moves the build to
the ipq40xx target specific build step. This change is needed to make
the firmware files show up in the buildbot images.

Fixes: 02db8a19cb8d ("firmware: add Intel/Lantiq VRX518 ACA firmware package")
Fixes: 07b0e6f3d9bc ("firmware: add Intel/Lantiq VRX518 PPE firmware package")
Fixes: 13eb1f564ad7 ("firmware: add Intel/Lantiq VRX518 DSL firmware package")
Link: https://github.com/openwrt/openwrt/pull/16031
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>