config: Fix recursive artifact resolution

A previous change altered the config normalisation pass such that
each artifact field becomes a dictionary containing multiple
properties rather than a single string. This was not fully taken
into account by the resolution step, which resulted in recursive
artifacts not being correctly resolved. Fixed by properly
constructing "artifact_lut".

Fixes: 8a368fb5 ("config: Allow artifacts to be omitted from final copy stage")
Signed-off-by: Yuliang Wang <yuliang.wang@arm.com>