checks: Do not omit nodes with labels if symbol generation is requested

Commit 4038fd90 ("dtc: add ability to make nodes conditional on them
being referenced") added the new /omit-if-no-ref/ directive to mark
nodes as eligible to be discarded if not referenced. The mechanism to
process this happens before the symbol generation phase. This means even
if symbol generation is requested and the node has a label, it will be
discarded if there are no references to it within the same file.

This is probably not what people expect. When using symbol generation to
compile base device trees for applying overlays, nodes with labels could
be referenced by the overlays, and therefore should be preserved.

Check if the node has a label and symbol generation was requested before
dropping the node.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Message-Id: <20190327035352.24036-1-wens@csie.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>