image_types_uboot: Don't truncate when assembling flash

The generate_flash_image step was creating a file then using dd to
fill in the pieces, but missed adding the conv=notrunc flag, so each
step was shortening the file to its output.

Add the forgotten conversion flag to each dd command.

Signed-off-by: Milton Miller <miltonm@us.ibm.com>
diff --git a/meta-phosphor/classes/obmc-phosphor-image_types_uboot.bbclass b/meta-phosphor/classes/obmc-phosphor-image_types_uboot.bbclass
index adb86e0..164d1ed 100644
--- a/meta-phosphor/classes/obmc-phosphor-image_types_uboot.bbclass
+++ b/meta-phosphor/classes/obmc-phosphor-image_types_uboot.bbclass
@@ -76,11 +76,11 @@
        dst="${ddir}/${FLASH_IMAGE_NAME}"
        rm -rf $dst
        mk_nor_image ${dst} ${FLASH_SIZE}
-       dd if=${ddir}/${uboot} of=${dst} bs=1k seek=${FLASH_UBOOT_OFFSET}
-       dd if=${ddir}/${kernel} of=${dst} bs=1k seek=${FLASH_KERNEL_OFFSET}
-       dd if=${ddir}/${uinitrd} of=${dst} bs=1k seek=${FLASH_INITRD_OFFSET}
-       dd if=${ddir}/${rootfs} of=${dst} bs=1k seek=${FLASH_ROFS_OFFSET}
-       dd if=${ddir}/${rwfs} of=${dst} bs=1k seek=${FLASH_RWFS_OFFSET}
+       dd if=${ddir}/${uboot} of=${dst} conv=notrunc bs=1k seek=${FLASH_UBOOT_OFFSET}
+       dd if=${ddir}/${kernel} of=${dst} bs=1k conv=notrunc seek=${FLASH_KERNEL_OFFSET}
+       dd if=${ddir}/${uinitrd} of=${dst} bs=1k conv=notrunc seek=${FLASH_INITRD_OFFSET}
+       dd if=${ddir}/${rootfs} of=${dst} bs=1k conv=notrunc seek=${FLASH_ROFS_OFFSET}
+       dd if=${ddir}/${rwfs} of=${dst} bs=1k conv=notrunc seek=${FLASH_RWFS_OFFSET}
        dstlink="${ddir}/${FLASH_IMAGE_LINK}"
        rm -rf $dstlink
        ln -sf ${FLASH_IMAGE_NAME} $dstlink