Use grub binaries from tools tree instead of from image

Use grub binaries from tools tree instead of from image

Let's give this another try and use grub tools from the tools tree
instead of from the image.

We also hardcode the grub prefix per distribution because if we use
grub binaries from the tools tree there might not be any installed
in the image itself which means we can't derive the prefix from the
binaries in the image.