refine get_kernel scripts.
  - added file existance checks.
  - added arch-separated downloading functions.
refactoring (not included behaviour changes)
  - Change script structure for arch-separated style.
  (get_ubuntu-core-12.04.3-core-i386_current.tar.gz.sh split to 3 files,
  per-arch/independent scripts.)
update install readme. fixed wrong instructions.
added "no download" behaviour.