image.py: add script output to the rootfs log

Let's add output of image creation script to the bitbake log
as it can contain useful information.

One good example of such an information is wic report about
artifacts and .wks file used for image creation.

(From OE-Core rev: fd6eba587e39142134aeb9044393c08a3f79d28c)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/image.py

@@ -11,11 +11,14 @@ def generate_image(arg):
             (type, create_img_cmd))
 
     try:
-        subprocess.check_output(create_img_cmd, stderr=subprocess.STDOUT)
+        output = subprocess.check_output(create_img_cmd,
+                                         stderr=subprocess.STDOUT)
     except subprocess.CalledProcessError as e:
         return("Error: The image creation script '%s' returned %d:\n%s" %
                (e.cmd, e.returncode, e.output))
 
+    bb.note("Script output:\n%s" % output)
+
     return None