Add cryptboot-grub-warning script

[ljfranklin]

• Place this script at /usr/local/bin/grub-install to raise an error
  telling the user to run cryptboot update-grub instead
• Little guard rail to prevent updating the bootloader and forgetting to
  sign the new binary
• Use absolute path to call grub-install in cryptboot script

[jugalnet]

I got locked out of my system because I forgot to run cryptboot update-grub after grub-install. This pull request addresses this exact issue well and imo should be merged in this branch.
Making this check a default would be a good idea in my opinion, adding the following to the PKGBUILD does the job.
mkdir -p "$pkgdir/usr/local/bin/"; ln -s "$pkgdir/etc/cryptboot-grub-warning" "$pkgdir/usr/local/bin/grub-install"

EDIT: Made a pull request with a few of my suggested changes