dietsplash: specify install rootdir

This takes care of rootdir being different when using usrmerge

Signed-off-by: Khem Raj <raj.khem@gmail.com>
(cherry picked from commit 8cec1f1fd9)
Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-oe/recipes-graphics/dietsplash/dietsplash_git.bb

@@ -17,6 +17,6 @@ inherit autotools
 S = "${WORKDIR}/git"
 
 EXTRA_OECONF = " --with-systemdsystemunitdir=${systemd_unitdir}/system \
-                 --disable-staticimages"
+                 --disable-staticimages --with-rootdir=${root_prefix}"
 
 FILES:${PN} += "${systemd_unitdir}/system/"