scripts: using extend in list_boards for variant list

Using extend instead of append to correctly extend the list of valid
board identifiers.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

scripts/list_boards.py

@@ -252,7 +252,7 @@ def add_args_formatting(parser):
 def variant_v2_identifiers(variant, identifier):
     identifiers = [identifier + '/' + variant.name]
     for v in variant.variants:
-        identifiers.append(variant_v2_identifiers(v, identifier + '/' + variant.name))
+        identifiers.extend(variant_v2_identifiers(v, identifier + '/' + variant.name))
     return identifiers