Don't fetch D-I images for the prep subarchitecture

prep has not been available for a long time.
Thanks to Ian Campbell for pointing that out.

debian/changelog

@@ -11,6 +11,7 @@ debian-cd (3.1.3) UNRELEASED; urgency=low
   * Only do boot disk stuff for x86 and alpha if the image is supposed to be
     bootable. Avoids downloading x86 D-I images for all 30+ CDs in a full set.
   * Don't download floppy D-I images for i386; they're no longer available.
+  * Don't fetch D-I images for the prep subarchitecture; it is not available.
 
   [ Steve McIntyre ]
   * Remove bashism in tools/apt-selection. Closes: #530972

tools/boot/lenny/boot-powerpc

@@ -83,7 +83,7 @@ if [ -n "$KERNEL_PARAMS" ]; then
 fi
 cp $BASEDIR/data/$DI_CODENAME/yaboot/ofboot.b ofboot.b
 
-for subarch in powerpc powerpc64 prep
+for subarch in powerpc powerpc64 #prep
 do
   case $subarch in
     powerpc|prep)

tools/boot/squeeze/boot-powerpc

@@ -83,7 +83,7 @@ if [ -n "$KERNEL_PARAMS" ]; then
 fi
 cp $BASEDIR/data/$DI_CODENAME/yaboot/ofboot.b ofboot.b
 
-for subarch in powerpc powerpc64 prep
+for subarch in powerpc powerpc64 #prep
 do
   case $subarch in
     powerpc|prep)