Draft: main/grub: add Grub code reinstall functionality

Whenever changes occur with the Grub package such as a new version or security patches or bug fixes to the existing version then the Grub code actually used for booting is likely to require updating.

Add a script, reinstall-grub, that can be run manually and also add to the Grub package trigger the ability to automatically run this script upon package update.

Also add a configuration file to contain the necessary settings in order for the script to be able to re-run grub-install correctly.

Automatic running of reinstall-grub is disabled by default and is controlled by a setting in the new configuration file.

The intention is that Alpine's setup-disk installation script will populate this configuration file with (some) of the necessary settings.

This MR will be followed by a separate MR to alpine-conf to add the configuration functionality to setup-disk.