The download link for unionfs 1.1.5 has changed. So the easiest way to solve the problem is to change the link in 'openembedded/packages/unionfs/unionfs.inc'

Just change the line beginning with SRC_URI = to:
SRC_URI = "ftp://ftp.filesystems.org/pub/unionfs/OLD/unionfs-1.x/unionfs-${PV}.tar.gz "

Because of a new version of unionfs, somebody moved the 1.1.5 version to OLD....