disk: add default minimum directory sizes

Currently, we only specify a minimum size for
- `/` (1 GiB), and
- `/usr` (2 GiB).

This ensures that
- a separate `/usr` partition is at least 2 GiB,
- `/` is always at least 1 GiB,
- if `/usr` it not a separate partition, `/` is at least 3 GiB.

We could (or should), in the future, make it possible for image types to
override this mapping as part of their default config, for example, if
an image type by default requires a larger `/usr`.

internal/disk/partition_table.go

@@ -38,6 +38,12 @@ func NewPartitionTable(basePT *PartitionTable, mountpoints []blueprint.Filesyste
 		}
 	}
 
+	// TODO: make these overrideable for each image type
+	newPT.EnsureDirectorySizes(map[string]uint64{
+		"/":    1073741824,
+		"/usr": 2147483648,
+	})
+
 	// Calculate partition table offsets and sizes
 	newPT.relayout(imageSize)