Add supernod build target.

superimage-nodeps and supernod depends
on images from $(ANDROID_PRODUCT_OUT) (not from
target files package). It doesn't rebuild source
images if they are present.

A typical workflow is:

    m -j
    # change code in system
    m snod -j
    m supernod -j

Test: For non retrofit, run:
      `m snod -j; m supernod -j`
Fixes: 128321505
Change-Id: Ib8c011cadb9c0cd334234aef39f19be6a48fee62
diff --git a/tools/releasetools/build_super_image.py b/tools/releasetools/build_super_image.py
index bb0e641..38ea3d6 100755
--- a/tools/releasetools/build_super_image.py
+++ b/tools/releasetools/build_super_image.py
@@ -24,9 +24,8 @@
   - target files package. Same as above, but extracts the archive before
     building super image.
   - a dictionary file containing input arguments to build. Check
-    `dump_dynamic_partitions_info' for details.
+    `dump-super-image-info' for details.
     In addition:
-    - "ab_update" needs to be true for A/B devices.
     - If source images should be included in the output image (for super.img
       and super split images), a list of "*_image" should be paths of each
       source images.