switchroot: Fix split source/build directory
switchroot: Fix split source/build directory

If you have split source and build directories, then building
static ostree-prepare-root fails to find the source files.

https://github.com/ostreedev/ostree/issues/1429

Closes: #1445
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree