Building Custom yocto image for WP8548


I have built yocto image successfully. And I am trying to flash kernel and rootfs using fastboot but while flashing rootfs getting following error

unknown partition 'rootfs’
error: cannot determine image filename for ‘rootfs’

I have flashed kernel image successfully.
kernel linked to boot-yocto-mdm9x15.img
rootfs linked to mdm9x15-image-minimal-swi-mdm9x15.2k.default

Please help me to flash rootfs.
If i reboot the system without rootfs, system is itself not booting.