extract_utils: cleanup variable names and arguments passed to extract() function

* This also makes the --section argument non-positional, since otherwise
  it is not possible to easily support more than one optional positional
  argument. This is in preparation of one more optional argument to come
  in a follow-up patch: --kang.

Change-Id: Ieb142e0854319defb9a278ab68cd4aeefd0fbdd5
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>