Add a function that takes a partition table and adds a root filesystem,
but only if one does not already exist.  A root filesystem is any
mountable where the mountpoint is `/`.

The function decides whether to create a plain partition, a logical
volume, or a btrfs subvolume based on existing partitions and volumes in
the partition table.

This function is not currently used because our base partition tables
always have root filesystems, but it will be part of the new custom
partition table generator.

Add a function that takes a partition table and adds a boot partition,
but only if one does not already exist.  A boot partition is any
mountable where the mountpoint is `/boot`.  Although the /boot
mountpoint must always be on a plain partition, this is not validated.

This function is not currently used because the NewPartitionTable()
function handles this automatically during layout conversions (in
ensureLVM() and ensureBtrfs()), but it will be part of the new custom
partition table generator.

Define a new const string: DosLinuxTypeID, which is used for Linux
filesystems on dos/mbr partition tables.

In the new helper functions, use the partition table type to determine
which ID to use.  This means that the functions will return an error
when the partition table type is not set (or is set to an unknown
value).

Update the tests to match the new behaviour and to test the correct ID
selection.