Bug 1410638. Currently, a theme that specifies no $theme->parent
in its themeconfig.php is forced to use "raw" as its parent.
With this patch, there are three options:

1. $theme->parent = false; : No parent theme.
2. $theme->parent = "sometheme"; : Sometheme is the parent
3. $theme->parent not defined at all: "raw" is the parent

Change-Id: Ibfeeae246b0f505b5759ee77eeed32467c464cab