fix compiling on Mac with non-GZIP kernel compressions

  * Non-GZIP compressions do not include the size in the file. Makefile.lib
    uses the stat command to determine the size of the file and then append
    the file size to the end of the image.
  * On BSD systems like Mac, stat does not recognize -c "%s" instead it
    needs -f "%z". This resulted in a non-booting kernel since the appended
    file size was either corrupt or 0.
  * Run both forms of stat, using whichever does not error out.

Change-Id: I95fd913074db90bb2b4afd8a2667246bea4a62e3