extract_utils: be compatible with system-as-root layouts

* The use case is that if you have the following layout:
  $TOP --- system.img
       |
       +-- vendor.img
  you should be able (from $TOP) to:
    mkdir system; mount -o ro,loop system.img system
    mkdir vendor; mount -o ro,loop vendor.img vendor
  and then (from device tree)
    ./extract-files.sh $TOP

  But this doesn't work if system.img is SAR and contains another
  "system" dir inside. This patch makes sure it searches for a "system"
  dir in the provided path as well, if it couldn't find the blob
  anywhere else.

Change-Id: Ib49cd5b587b3a57478a66ff69cf840270c2b1403
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>