wic: bootimg-partition: Fix file name in debug message

Debug message about using custom configuration file includes file name
with incorrect extension. Correct file name to "extlinux.conf".

(From OE-Core rev: bb063e8b461b8c3c0b5b6965f26a555781dc94b7)

Signed-off-by: Daniel Semkowicz <dse@thaumatec.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit dd63e1520454b2d53a48b72eaae126059af9809b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

scripts/lib/wic/plugins/source/bootimg-partition.py

@@ -112,7 +112,7 @@ class BootimgPartitionPlugin(SourcePlugin):
                 # Use a custom configuration for extlinux.conf
                 extlinux_conf = custom_cfg
                 logger.debug("Using custom configuration file "
-                             "%s for extlinux.cfg", configfile)
+                             "%s for extlinux.conf", configfile)
             else:
                 raise WicError("configfile is specified but failed to "
                                "get it from %s." % configfile)