uboot: fixes to uboot-extlinux-config attribute values
The way this class uses overrides to support generation of multiple
sections is subject to two different issues: 1) labels that conflict
with existing override names causing the value for the conflicting label
to be set for all labels, and 2) reusing the override list through each
iteration, prepending each new label to the list of overrides makes
earlier labels' value take precedence over later labels, making later
labels virtually impossible to customize.
The first issue is resolved by removing all label names from overrides
before iterating over labels. The second issue is resolved by
generating a fresh list of overrides with only the current label added.
The current label is also appended to the list of overrides instead of
prepended, which makes it the highest priority override. This is
matches the behavior of devtool-source.bbclass, which similarly
monkey-patches overrides.
