releasetools/release.sh: cleanup & enhancements

 - use the proper  make variable to ignore file lists
   SLOPPY_FLIST instead of CHECKFLIST, which was a minix hack.

 - Add BUILDOPTIONS such that the users can activate optional
   features without editing the script.

Change-Id: Id731db96c38c2118c4c58e007f2804008a7a893f