Move remaining overlay helpers to overlay.{c,h}

Move remaining overlay helpers to overlay.{c,h}

Move
	- ovl_get_rootfs_dir()
	- mount_entry_create_overlay_dirs()

from conf.h to overlay.{c,h} where they belong.

Rename
	- mount_entry_create_overlay_dirs() --> ovl_mkdir()

in accordance with the ovl_ prefix naming scheme for types and functions
associated with overlay.

Take the chance to add whitespace between operators where missing.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>