api_examples/Makefile: General cleanup

api_examples/Makefile: General cleanup

* Remove symlinking of files located outside api_examples/

* Auto generate dependencies for files located outside api_examples/

* Update names of variables to be similar to those in tools/Makefile

* Fix out of tree build error
  Dependencies are calculated for all files in the SRCS variable.
  Previously, the SRCS variable contained files which were symlinked
  into the api_examples/ directory.  These symlinked files did not exist
  when dependencies were calculated when building out of tree.  This
  resulted in errors such as:
    make[1]: *** No rule to make target `/work/wd/tmp-ppc/api_examples/.depend', needed by `_depend'.  Stop.
    make[1]: Leaving directory `/home/wd/git/u-boot/work/api_examples'
    make: *** [depend] Error 2

  Since symlinked source files are no longer used, this bug no longer
  exists.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Acked-by: Rafal Jaworowski <raj@semihalf.com>