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