imagebuilder: exclude metadata for profiles that have no kernel

imagebuilder: exclude metadata for profiles that have no kernel

Device profiles that specify 'DEFAULT := n' are being included
in the imagebuilder metadata, specifically in .profiles.mk, even
though there is no kernel built for the device.  This results in
'make info' showing the device as valid, but then 'make image
PROFILE=xxx' failing with 'No rule to make target xxx-kernel.bin ...'

We exclude these profiles from the imagebuilder, avoiding these
errors.

Fixes: https://github.com/openwrt/openwrt/issues/18410
Signed-off-by: Eric Fahlgren <ericfahlgren@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18748
Signed-off-by: Robert Marko <robimarko@gmail.com>