Introduce a new hook for builtin_curthooks to configure the system to
boot. This currently uses grub but will also support extlinux in future.
The configuration key is 'boot'. It can be thought of as a rename of the
existing top-level 'grub' key, except that it allows one or more
bootloaders to be installed, using a new 'bootloaders' key. So the old
configuration:
The 'bootloaders' key is required. It does not have a default. This is
to ensure that each use of Curtin is converted with due care to which
bootloaders should be supported.
If 'boot' is missing, then a top-level 'grub' key is converted into
'boot', with all properties remaining the same except for the automatic
addition of:
bootloaders:
- grub
This maintains backwards compatibility with the old schema.
It would be quite nice to use an enum for the bootloaders.
Move handling of grub_install_devices to setup_boot()
Apart from in tests, setup_boot() is the only caller of setup_grub(), so
handle the conversion setup_boot(), to avoid setup_grub() messing with
the config.
In preparation for supporting bootloaders other than grub, create a new
setup_boot() function and call that instead of setup_grub(). Move
calling uses_grub() into this function.
This will allow setting up another bootloader later.
This is not used by the main program and it is probably not a good idea
to start using it. The functionality of this module is accessed by
calling the install_grub() function.
Remove this unused code.
Leave this in the commands/ subdir for now, but we could consider moving
it, e.g. into a new boot/ subdir.
