Secure Boot: update grub and shim binaries

Secure Boot: update grub and shim binaries

Quoting János Pásztor from https://github.com/grml/grml-live/pull/129:

  After 581da7443c68c362a7677c905ab5c63eb23c5b73 and using the `debian`
  style secure boot grml will not start on machines with secure boot
  enabled, but fails with a `signature verification error`

  After some investigation it turned out that we hit
  https://bugs.debian.org/925550 with our boot binaries.

  I have updated them from debian and managed to boot with them
  properly.

While at it, switch from http://ftp.de.debian.org/ to
https://deb.debian.org/ which has proper SSL certificate available.

Thanks: János Pásztor <model87@freemail.hu> for bugreport and PR