prepare-bootpartition: fix checking of result

Fix missing invert of exit code for "sed" command to correctly return error.

prepare-image/prepare-bootpartition.sh

@@ -86,7 +86,7 @@ done
 #======================================================================
 # The sed pattern replaces the partitions only when the line starts with
 # BOOTPARTITIONS=
-if sed  -i /usr/libexec/prepare-common.sh \
+if ! sed  -i /usr/libexec/prepare-common.sh \
     -e "/^BOOTPARTITIONS=/s|/by-label/LINUX *|/by-label/$PARTITION_LABEL |" \
     -e "/^BOOTPARTITIONS=/s|/dev/mmcblk[^\"]*|$BOOTPARTITION|"
 then