bug: if dtb isnt define, dont set it to blank

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>

lib/debian-finish.sh

@@ -159,7 +159,7 @@ echo "cape_enable=" >> ${wfile}
 if [ ! "x${dtb}" = "x" ] ; then
 	echo "dtb=${dtb}" >>  ${wfile}
 else
-	echo "dtb=" >>  ${wfile}
+	echo "#dtb=" >>  ${wfile}
 fi
 
 if [ ! "x${optargs}" = "x" ] ; then

lib/ubuntu-finish.sh

@@ -129,7 +129,7 @@ echo "cape_enable=" >> ${wfile}
 if [ ! "x${dtb}" = "x" ] ; then
 	echo "dtb=${dtb}" >>  ${wfile}
 else
-	echo "dtb=" >>  ${wfile}
+	echo "#dtb=" >>  ${wfile}
 fi
 
 if [ ! "x${optargs}" = "x" ] ; then

mk_mmc.sh

@@ -1116,7 +1116,7 @@ populate_boot () {
 		if [ ! "x${dtb}" = "x" ] ; then
 			echo "dtb=${dtb}" >>  ${wfile}
 		else
-			echo "dtb=" >>  ${wfile}
+			echo "#dtb=" >>  ${wfile}
 		fi
 
 		mmcargs="mmcargs=run message; setenv bootargs console"