extract_utils: extract(): fix src not being searched in "src:dst;args" spec

* Root cause of the issue is improper naming of variables. Turns out,
  there was no variable to even denote where the blob should have been
  searched for, at "src".
* Previously there was one such variable, suggestively called "TARGET", that
  was desperately trying to serve as both, depending on who +2d hacks harder.
* One such example is "c982836c extract_utils: Fix makefile generation issues".
  That patch deliberately trimmed the "src:" from a spec (therefore
  obviously breaking the search at src) but enabling the searching at
  dst, via the good-for-all TARGET variable.
* This patch introduces the following variables:
  - SRC_FILE: absolute path corresponding to SPEC_SRC_FILE in the
    Android filesystem.
  - DST_FILE: absolute path corresponding to SPEC_DST_FILE in the
    Android filesystem. Somewhat analogous to the old TARGET variable,
    but actually contains the leading / as well (/system/bin/adsprpcd
    instead of system/bin/adsprpcd).
* Use existing common get_file() function (which previously was
  impossible to use, because it was impossible to determine calling
  arguments properly) to reduce complexity of handling adb and disk
  image as blob sources.
* Via the new SRC_FILE and DST_FILE variables, search for a blob in all
  possible locations (including paths stripped of "/system" which
  transforms an absolute path in the Android filesystem into a proper
  relative path to that file in a disk image).

Change-Id: Ic40fb4dc93541d8b3f33fde586b773199cf4ded2
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>